Gladstone is a relative of Donald and Scrooge, another duck.
As for Pete… it’s really hard to tell.
At first he was bear, see the pre-Micky stuff like the Oswald cartoons, but then made a cat when Micky came about, then at some point when they started to make him an antagonist for a number of other Disney character and they kind of dropped having any indication of what he is, about just before they paired him with Goofy for Goof Troop.