Can I use gel pens on acetate sheets?
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.