I want to try some fender flatwounds. They come in three 'thicknesses' or gages.

This is the first set of strings I"m buying so have no point of reference or experience to suggest one over the other.

Fairly new to playing, just learning so not trying to dupe a sound or play a style yet. From what I've read and what I hear I want to try flats, so the only question is what gage is a good one to try/start with?

My used GSR200 came with strings of a round nature but I know nothing else about them, they have blue silk wraps and I assume (based on price and that store) they were cheap ones. I really detest the sound of the G string.
When it comes to string you just have to try it out. There is no defult string gague that every bassist will like. Ive been using the same brand and gague of strings for five or six years but that was after switching brands and gagues every two months til I found something I liked. Try the mediums and if you want some lighter or heavier make the move from there.
i'd say start with 'standard' or medium gauge strings. that way you'd have more of an idea whether you'd want thicker or thinner.