Now this is a loaded question - LOL
1st thing you need to decide is what kind of TGP you plan on making - one with a lot of categories or one that's niche specific & if so, what categories for that niche will you have. Example: if you decided on an Asian only TGP, would it jsut be all "asian galleries" or sould you have asianbabes, asian amateurs, asian hardcore, asian cumshots, etc.
Then you need to build the site - most have warning page, then the main page where all the galleries are located & some also have an archive of old galleries (sorted by category if that's the path you choose from the 1st part)
Then you need a script for it, as running it by hand is a pain in the ass.
Then you need galleries for it until you can build up a decent amount of submits. You can fill it with sponsor hosted galleries, but I know that as a webmaster, if I see a TGP that's all sponsor hosted galleries, I tend to think it's a hub & not a "legit" TGP for webmasters to submit too. So I'd say find galleries on other TGP's that you like & that fit into the rules that you have for your TGP & list those - just make up your own descriptions & don;t copy what the other TGP has already posted.
After that, you "open" it, set up some regular link trades & ask for submits from other webmasters. Submit the TGP to webmaster resource sites (like this one) & post on boards about the site - but don't spam! Looks for threads where people are asking for -places to submit & if you site fits, post