GURPS Banestorm New World Hexcrawl
GURPS Banestorm clearly states that the form practiced in the known setting is the basic GURPS system, but there are a lot of options for magic in 4e - Sorcery, Ritual Path Magic, Powers-as-Magic, etc - that did not exist when Banestorm came out.
So what I'm going to do:
The Old World: Basic GURPS magic,perhaps using styles from GURPS Thaumatology: Magical Styles.
The New World: Maybe something else, you'll have to find out in play.