GURPS M:tA

[Terwin]

If you want to use the power structures from M:tA, why not just use Detect, Control, and Create?

1 dot is usually Detect
2 dots is usually a limited Control
3 dots adds in limited Create/Transform
4 dots removes many of the Limits on Control
5 dots removes many of the limits on Create/Transform

All of these of course have accessibility limitations based on the Tradition of the user(Generally Preparation and appropriate materials for the tradition), and the rules of consensual reality(Paradox and such)

You can either use the M:tA spheres, or define your own.

You can even have power levels independent of the sphere level by letting different Mages take different levels of the create/control powers

'Magic' in Mage the Ascension was less about 'magic' as used by Merlin, and more about exerting your will on the reality around you like a minor deity.
Seems like the best way to emulate that in GURPS would be to use the 'god-like' create and control powers.