For the noun גר (stranger), the essence of the word is that one lives in a land, near or far, that is not his own. And the Canaanites who accepted upon themselves the seven commandments of Noah are called גר תושב, while anyone who comes from afar and converts to Judaism is called a גר צדק. And both of them can be called by just the name גר.

And it is always learned from its context whether it means a גר צדק or a גר תושב.

Or through the repeating of the noun גר twice, once for a גר צדק and once for a גר תושב.

Or through היקש (analogy), etc.