Draw in full color (RGB for example), and only start the compression and formatting etc once your work on the painting is finished.
Make sure you're using no dither, and making the indexed color mode be 256-bit.
After that the file is saved by the program, so unless you've done something with the drawing tools pre-save that affected the 176,176,176 background, you shouldn't have anything but a 176,176,176 bg.
You can double check the grey bg's RGB value is correct with an eyedrop tool for example. Open the file as it is without doing anything to it of course.
