If you scale up your image in GIMP, which it looks like you might have done, GIMP automatically rasterizes all of your text layers. If you catch it early enough, you can use the text tool on those "formerly" text layers which gives you the option of editing them as text again to increase the font size normally. It all depends on your saves and timing. Likely at this point, you'll need to reenter all of your text either through GIMP, or as tilt suggested, Inkscape.