Almost all TGPs/MGPs require a Recip (Backlink) these days, so if you want to submit to 300 sites, you'd have to add 300 Recips to your galleries. Most webmasters use 3 Recips per gallery, so you'll end up with 100 gallery mirrors, like:
yoursite.com/galleries/index.html --> original gallery
yoursite.com/galleries/index1.html --> mirror gallery with the first 3 recips
yoursite.com/galleries/index2.html --> mirror gallery with the second 3 recips
yoursite.com/galleries/index3.html --> mirror gallery with the third 3 recips
.
.
.
yoursite.com/galleries/index101.html --> mirror gallery with the last 3 recips
But most better gallery submitters do that automatic these days, I'd suggest you take a look at the gallery submitting tutorial I've recently written (
http://www.gofuckyourself.com/showthread.php?t=573984)