I want to trace the outline of buildings on a printed map onto something transparent, to then put on black paper. Will the ink from white gel pens stick to sheets of acetate? Or will I have to use a different transparent material?
Acetate is waterproof, and gel pens work by having pigment suspended in a water-based gel. So, intuition is that if the pen applies at all (it may not even grab to the surface), then it would be heavily prone to smearing.
An alternative could be vellum, which is often used just as you described: to trace something on a nice surface that can be used for something else.
Most ball-point pens of any kind need a little bit of tooth in order to release their ink onto a surface. I just tried a gel pen on several plastic films and it didn't work. It does work on vellum, though. If you want to use the acetate, I'd go with a fine-point Sharpie. It can be rubbed off many smooth surfaces, but it will apply ink. Acrylic pens would likely work, but they're a bit touchy and irritable to work with when you apply them over a smooth surface.