such keypads can be used for just about any sort of game. you could use them for all genres if you program them in the right ways to be helpful.
i have a tartarus i picked up for those games which felt a bit awkward for keyboard yet did not have controller support. i played the original guild wars 1 for 6+ years with keyboard only (i used directional keys and numpad mainly) but do not own gw2 (since it is not the spiritual successor its a wow clone now) so cannot tell you specifically for that game. i did use it for tera (though preferred a controller instead for that one), dragons prophet and c9 where it came in useful though i'm so used to using keyboard input its a little odd for me to use.
if you do use a tartarus i'd suggest using it often instead of swapping to it for only certain games as if you use it often your fingers will get a memory to them and remember where keys are. otherwise, you might have to think a bit more when you go to use certain keys.
overall its good, but could be better (games and gamepads NEED analog movement sticks and support!!!) but worth having if you find keyboard control awkward.