The
MyGurps website has just what you want, as the article
Upgrading the Magic System. You want the section "Modifying Spells" and you should look at "Long Range Spells" as well, which is a worked version of it.
The Author of MyGurps, Reverend PK, had a stint as the assistant line editor for Gurps and is a published Gurps author. The site is pretty good.