Showtime are paying GBP to show their fights. They could easily pay another promoter to show their fights if they wish. Unless the other promoter is already contracted to another tv channel.
Well if there is another fighter who is exciting I am sure he could get on, networks only care about $$$
Top Rank and GBP are pretty much the only two promoters. Others would be allowed on their networks, but there's not many fighters not aligned with one of these two.
Goosen has popped up with Lopez (though he is managed by Haymon), arreola will be on it (though again haymon) and Molina (that was more for bey and Tmt though)
I don't know that it's an exclusivity deal, but certainly Top Rank is expressly unwelcome to put their stuff on Showtime - just as GBP isn't on welcome on HBO. (it used to be the other way around)
There are a lot of promoters who have fighters on both networks. GBP and Top rank are the biggest but there are several top fighters who are not with either of them. COTTO is one I know for sure is a free agent.
Only Top Rank is banned from Showtime. GBP's stable is so large that they can fill up all of Showtime's dates though