Adjusting gif framerate in GIMP 2

How can I open a gif with GIMP and adjust the framerate?

When I try to reexport it, the option for this is whited out (see screenshot below):

Solution 1:

Just click on the As Animation checkbox - it will enable the animation options (and actually save an animation, instead of a flatenned GIF image)

That is your answer - but taking the opportunity of the question repeat an information that is somewhat hard to find or understand: if you want to customize the frame duration of any specific layer, it is possible to do that by putting a special sequence in the given layer's name within GIMP: on its name, include the frame duration in milliseconds, post fixed by "ms". That is, if you want the last (topmost in GIMP) frame to stay on the screen for 3 seconds, just add (3000ms) as part of its name. (Add the parenthesis as well - they are not needed for gif export, but other plug-ins may require them).