To give you some background on how to make your mask usable with GIMP:
The image you provided the link for is in RGB to begin with.
So you need to change it to indexed. (Clicking: Image>Mode>Indexed)
Even if you choose 256 colors it automatically runs down the colormap to the colors present in the image (2 colors in your image).
So to fix this afterwards I create a new colormap (Colors>Map>Set Colormap) which includes exactly 256 colors. I pick one out of the list which includes a lot of different colors.
Now I also want black to be the transparent background color. AGS uses the color with ID 0 as transparency.
So I open the color map (Windows>Dockable Dialogs>Colormap), right click it and select Rearrange Colormap...
Now I move the black color to position 0, and the colors I want to use to positions 1-15 on the palette. Those are the colors AGS is going to use for Walkable Areas 1-15 (and that you should use to draw the walkable areas, obviously).
That's how you make it usable for AGS.
Note that when you export a mask from AGS and then import it into GIMP, GIMP should never change the color format without asking you.
So you probably changed some settings that made it not work in the first place. Otherwise you could have just exported and used it in AGS.
Also when exporting the image to a new name you have to make sure that both "Run Length-Encoded" and "Do not write color space information" are not selected in the pop-up window.
AGS is very sensitive as to what it accepts to import masks, so I hope that gives you a better overview on how to use GIMP for that
The image you provided the link for is in RGB to begin with.
So you need to change it to indexed. (Clicking: Image>Mode>Indexed)
Even if you choose 256 colors it automatically runs down the colormap to the colors present in the image (2 colors in your image).
So to fix this afterwards I create a new colormap (Colors>Map>Set Colormap) which includes exactly 256 colors. I pick one out of the list which includes a lot of different colors.
Now I also want black to be the transparent background color. AGS uses the color with ID 0 as transparency.
So I open the color map (Windows>Dockable Dialogs>Colormap), right click it and select Rearrange Colormap...
Now I move the black color to position 0, and the colors I want to use to positions 1-15 on the palette. Those are the colors AGS is going to use for Walkable Areas 1-15 (and that you should use to draw the walkable areas, obviously).
That's how you make it usable for AGS.
Note that when you export a mask from AGS and then import it into GIMP, GIMP should never change the color format without asking you.
So you probably changed some settings that made it not work in the first place. Otherwise you could have just exported and used it in AGS.
Also when exporting the image to a new name you have to make sure that both "Run Length-Encoded" and "Do not write color space information" are not selected in the pop-up window.
AGS is very sensitive as to what it accepts to import masks, so I hope that gives you a better overview on how to use GIMP for that
