This is very simple: after you register to the traffic trade system, when you send visitors to gallery-dump (if you run a TGP, you force hits or force the link's position, if you own a blog, you place a jscript or hardlink on your site), gallery-dump sends visitors back to your website, the same amount or more.
The list of partner websites cannot be made one kilometer high, so there is a limit to how many websites are listed.
No tricks here: those who send the greater number of visitors appear on top, those who really don't send enough stop being listed (and sorry for them, but it's nobody's fault, there has to be a limit to the number of available spots.)
In the past, gallery-dump had been using Xclicks for its linktrade system, and it had been working well.
However, sadly, the quality of service of Xclicks has gradually fallen. There have been more and more downtimes, the admin(s) stopped responding to bug reports, the development of new features ended at some point in 2013 (it's old already), their support board has shut down... Actually, my greater surprise is that Xclicks is still working to this day! O_o
This is why Xclicks is, essentially, replaced by the Trade Expert 3 system, a widely used and trustworthy system.
Look in the sidebar of gallery-dump: FIRST it's TE3, and SECOND, below it, receiving many less clicks, it's Xclicks, that is only kept for the old partners.
If you're a webmaster, and you still have an Xclicks linktrade with gallery-dump, please sign up to the TE3 linktrade system, eventually Xclicks will disappear!