I think
Transhuman Space had rules for radiators in space in Appendix A, but you'd have to convert the simplified spacecraft design system there into GURPS Vehicles terms.
Edit: I've just checked my copy, it does have rules for radiators. They are (obviously) tied to the Transhuman Space design system so you'd need to do a bit of work to convert, e.g. working out what an "old fission power plant" is in Vehicles terms.