There's also
Aseprite for Linux. One thing is that you cannot resize the window on it while the program is running unless you build it yourself. You can change the ~/.asepriterc so it boots larger, but again it can't be changed while running. Or so I've been told. I... never bothered using a native build, I just used it through wine the few times I messed with Linux. Standard other warning: For whatever reason the dev likes it to use 2x2 pixels by default. To change this, open ~/.asepriterc and change ScreenScale = 2 to ScreenScale = 1.
Anyway, Aseprite's my main software regardless, I like it better than Graphics Gale and everything else I've tried. I dig the simple, single window interface. Supports animation and is free even if you're saving gifs. But of course... your mileage may vary, because basically no one I've recommended it to likes it enough to use.
(Reason for the window trouble: It uses a modified Allegro library that allows window resizing, but for some reason the build you'd get if you searched for it in say... Linux Mint's software manager is built with standard allegro.)