Telling genders is supposed to be easy when they are newborns, cause they have no fur.
After the fur grows it's quite hard seeing the genitals, but there are some differences and you can figure it out. Of course it's not 100% sure, but almost.
I looked up some ads.
I am following some Facebook groups dedicated to adoption of rodents, because some people may want to give up theirs for some reason. (Some reasons are better than others, humans are sometimes a bit flimsy.)
But in the end I found a person who had two pregnant gerbils and was looking for new homes for the babies. I'm adopting one of those babies after it's weaned
There are some people who breed them because it's nice having little babies at home, and because they are super cute, and because you want to see which fur color you're getting, etc.
Personally I would not want to breed them, but I realize they can be so cute.