If you imported a GIF file into GIMP, your problem is that your image is in indexed mode.
In this mode, most filters and color tools won't work, and be grayed out (up to GIMP 2.8 - expect news on this front on GIMP 2.10).
So, all you have to do there is to go to
Image->Mode->RGB, and the Color to alpha menu entry will be enabled.
I don't think that will resolve your problem there: Color to alpha is a sophisticated plug-in to remove a color in a way that if the image is put over a solid background with the same color that was removed, it will look exactly like the initial image again: for that, it does not remove just the places where that color appear in pure form (say (0,0,255) green) - but it makes all colors that take green in their composition partially transparent.
My advice would be to use the Select By Color option to select the color you want to make transparent. If it is a single indexed color, instead of various shades of it, you can set the
threshoçd parameter to 0 - otherwise, experiment with the
select by controls until you mark all parts of the image you want transparent. After that,just perform
Edit->Cut (don't forget to add an Alpha channel to your layer before doing that
Layer->Transparency->Add Alpha Channel), and export your image as a GIF file. You won't need to convert to RGB and back to indexed mode if you just want to add alpha, and choose this method - it should work with the image in Indexed Mode.