What if magic returned? What if it never went away? How would magic interact with the post-apocalyptic world? Are magic and technology antithetical or are they capable of enhancing each other? The most important thing to figure out is probably what magic system to use – GURPS has a lot of options to choose from. GURPS Magic contains a detailed, flavorful system, but has many potentially game-wrecking spells. The most balanced approach is probably Sorcery, as the spells will be built the same way as psionics and mutations… but it may give the feeling that everything is the same. Personally I like Path Magic from GURPS Thaumatology (p. 121-165) – it is reminiscent of “classical” magic without requiring as much GM oversight as e.g. Ritual Path Magic.
As the Primary Cause
As a Secondary Effect
As mentioned above, you need to discuss the relationship between magic and technology. If they are opposed to each other, consider adding the Nature Power Modifier (GURPS Powers, p. 28) to Magery or the magical powers you build. However, this can make magic almost useless in the most interesting areas (such as radioactive wastelands). Why would a magic-user even enter such areas?
If you use Path Magic, the question of magic and technology boils down to whether you allow the Path of Gadgets (Thaumatology, p. 145-148). With magic and technology opposed, you should probably not use the path – the only appropriate ritual is Gremlins, and that can be cast using the Path of Luck. With magic and technology working together, the Path of Gadgets becomes a good way to either complement or supplant a Techie in the group. The only rituals that should be banned are Fuel and Unlimited Ammo, both which negate the resource-management part of the setting. If radiation is a factor in the setting, you'll also want to devise a ritual that allows you to deal with it. Alternatively you can allow the Dose (Thaumatology, p. 148) to clear away as many Radiation Points as the margin of success (for Effect Shaping) or energy accumulated.