I don't know if you can set the gamma value within gimp, but you can use an existing trick-image as a template instead.
(copy-pasting from an old thread:)
lazymode: find an existing image that has the gamma value you want
-open it in gimp
-"file>open as layers", choose your two pictures.
-resize image if you need to ("image>canvas size") and delete the original layer.
-right click on top layer, "add layer mask"
-"filters>render>pattern>checkerboard", 1 pixel wide
-click on bottom layer, "colors>levels", set "output levels" minimum value *very* high (very pale, close to white)
-click on top layer, "colors>levels", set "output levels" max value a bit lower (slightly dark)
-"image>flatten image"
-"file>export as", stupidshit.png, check the box next to "save gamma"
