There are loads of tutorials such as this available on how to use GIMP to create & edit sprite sheets.
There is even a plugin to help called Narly Sprite.
Exported as a grid sprite sheet:
Flattened and exported as an animated gif:
I also noticed that one of the stretch goals for the current Krita funding campaign on kickstarter is to add sprite sheets to it so if you prefer working with a tablet and are willing to help fund development you could chip in, (N.B. They have reached the base goal and people who hit some funding levels get to prioritize the stretch goals.)
On the other hand if you already have your sprite set in any of a number of standard image formats, (such as gif), you can load it in gimp and use the G'MIC Array & Tiles -> Montage filter to automatically produce a grid of your images or you can select the images from individual layers to copy and paste, or to use as a brush, in another image.