Alright, so we're all familiar with the dreaded unreliability of sorcery in my campaign world. It hasn't been too much of a problem in Orcunraytrel, because Orcunraytrel is different. But I'll give you a few examples. One time Eric was casting a healing spell on someone (I think he was actually trying to heal himself), the spell misfired. Instead of healing his target, his character vomited up several glowing pus colored darts of magic that whipped around and zapped him in the face, dealing damage to him instead of healing. Another time, the group was trying to use gaseous form to sneak inside of a building, and Fred's casting misfired, ended up raining a rain that turned everything it touched into a gaseous form version of itself. It worked out in the end, but it could have gone very bad. In the final battle of last campaign, Nakmander the Big Bad Evil was going to hit one of the players with power word: kill and it misfired and removed all the oxygen from the area and caused him and several others to pass out. He ended up with a blade in his face. Thank goodness for contingency.
Anyway, the way magic works in Pathfinder Hekinoe is that each spell has a 4% chance per spell level to misfire. 0-level spells have a 1% chance to misfire. There are some other factors, but that is the base for most spellcasters. Sorcerer type classes roll an extra d4 with the result increasing their caster level for the spell and also increasing the misfire chance by an equal amount. If a spell misfires, there are a bunch of charts to roll on that determine the end effect. The spell could just fail, or it could fail and misfire, work and misfire, or become overpowered and get a metamagic feat for free. It is pretty varied. There are also feats, traits, and flaws that can impact the misfire chance as well, but never reduce it to 0%. Magic items also have a percentage chance based on spells used in their creation and the length of time they have existed that can cause them to explode into fiery shrapnel when used.
The way magic works in GURPS is very different than in Pathfinder, no spellbooks or spells per day or anything like that. It is all based on the IQ stat and fatigue points. It is also skill based. Basically you treat each spell like a skill and buy it at a certain level (IQ+0, +1, -1, etc.). Spells also have prerequisites. For instance, you have to know create fire and shape fire to learn fireball. You also have the Magery advantage. Magery 0 is your basic magical sense, it allows you to determine whether or not something is magic in nature. You cannot learn spells without having Magery and some spells require a certain level of Magery for you to learn them. Magery above level 0 also increases your effective IQ for purchasing spells, for instance someone with Magery 2 and an IQ of 11 that buys wither limb at IQ+0 rolls at a skill level of 13 and not 11.
To cast a spell in GURPS you roll against your skill level (3d, roll lower than your skill = success, 3 or 4 equals critical success, 18 equals critical failure) and pay fatigue points, your fatigue points are determined by your HT stat, but can also be increased independently from HT. Spells also have a casting time. Often times you can put extra FP into a spell to increase its effectiveness. Fireball for instance costs 1 FP and takes 1 second to cast, however you can put in 1 FP a second for up to three seconds. Each FP spent deals 1d fire damage. So for three FP and casting for three seconds you can create a fireball that does 3d damage. When you cast the spell, you make a check against your fireball skill level and if you roll below your skill level you have successfully cast it. If you roll an 18, that is a critical failure and the spell doesn't work and you roll on a critical failure chart. If you roll a 3 or 4 (or up to a 6 depending on your effective skill with the spell), the spell is a critical success and costs no FP and does something additional up to the GM. You can also burn HP to cast spells as well. There is also something called mana, that is the ambient magical energy of the area. It can do things like make spells impossible to cast, or allow you to cast some for free.
Alright, so now you have all the background info. The most easily explained way that Hekinoe magic differs from basic GURPS magic is that it is based on Will and not IQ. My reasoning for this is because while magical rituals and incantations are learned, there is also a very strong focus on imposing your will on the force that is sorcery. Sorcery is wild and unpredictable in Hekinoe and it takes a strong willpower to force it to behave itself and do what you want. Because Will is based on IQ, it still favors intelligent creatures, but Will can also be bought for cheaper than IQ. It is conceivable that someone with an IQ of 9 could spend those bonus points to give themselves a Will of 13. However, it should be noted that there are a shit ton more IQ based skills than there are Will based, so the brain dead spellcaster is probably not the wisest decision. Especially considering most magic oriented skills are based on IQ.
The other way that magic differs from regular GURPS is that Hekinoe uses something called Threshold Limited Magery. Basically, you don't pay HP or FP for spells anymore. Instead, you track how many you would have spent and call it your tally. You keep a running total of your tally as you cast spells. Each spellcaster has something called a threshold, something like 30. Your tally also heals over time, something like 8 each day or so. Now, you can cast spells as normal (based on Will and making spell skill checks) as much as you want whenever you want (up to something like a tally of 150 or so). No FP or HP lost to the art. However, on any turn that you go over your threshold or cast a spell while your tally is over your threshold, you make a calamity check. This check is 3d +1 for every full five points your tally is over your threshold. These calamities can be pretty bad, a 3-10 on the roll does nothing though. As you move up the chart you see stuff like your threshold decreases for a few weeks, but you don't know by how much. You start losing advantages or gaining disadvantages, you start losing levels in your spells, or you can alter magic on a global scale. It can get pretty nasty and the one calamity that increases your chance of critical failure with spells can make things pretty frustrating. There are of course a bunch of advantages and disadvantages to modify your threshold and the rate at which your tally recovers. There are also ones that can modify how calamity prone you are.
So that is kind of the run down on how magic would work in Hekinoe with the GURPS system. Luckily there are a lot of different critical failure tables for spells for me to play around with. The system that was so difficult and time consuming to institute with Pathfinder has been almost a complete breeze to make work in GURPS.