So to resume:
1. what is my advantage of having other people posting their galleries?
Variety of content and a recip link back to your tgp coming from another website, that is how the established tgps are able to build massive serps
2. Which TGP software is the best on the market now and how to resize thumbs to 175x175 because the [Thumb Cropper] doesn't do anything in comusthumbs script.
smart thumbs and site organizer does this, smart thumbs have more options while so is simple
i have some
simple smart thumbs screen shots here
3. Would that be OK instead of designing a tgp just put it as an IFRAME inside another sofrware like I did here
no idea..