GURPSよろず

p.102
　Normally, enhancements and limitations are additive: add them together and apply the total modifier to advantage cost. For instance, +20% in enhancements and -50% in limitations come to -30%, so ability cost is 70% of advantage cost.

　Optionally, the GM may treat enhancements and limitations as multiplicative.
Total the enhancements and apply them first. Then total the limitations (reducing any total over -80% to -80%) and apply them to the result. In the above example, +20% in enhancements would increase ability cost to 120% of advantage cost, and then -50% in limitations would reduce this to 60%.

　The GM decides which model to use. The results aren’t the same (although they’re close for small modifiers), so using both isn’t recommended.
The additive model is a good “default,” but the multiplicative model can be fairer in campaigns where huge enhancements (like Cosmic, +300%) occur routinely.