Even if the two words were generic to begin with, if someone put them together in a unique way, then went to the trouble to build a successful website and brand name, then it wouldn’t be appropriate to register a very similar domain name.
“Pretty” and “Gambler” are both generic words, but once someone builds a website, generates significant traffic to that website, people recognize the site and type the domain name in to get to the site, then it’s effectively trademarked. So registering “prettygamblers.com” probably wouldn’t be a good think to do if someone had already worked hard at branding “prettygambler.com”.
The defining question in these kinds of issues is whether or not you might create confusion in the marketplace.