Hm, let’s see. Yes, if it’s looking for the CD, it’s the game.ins file you need. GoG are using weird iso file extensions, but they’re basically just iso files. After all, Dosbox must be able to access them.
Can you change to drive D: in Magic Dosbox?
Maybe another solution: I take it you preinstalled the game on your PC? GoG installs games to a subfolder, but mounts that folder as “C:\”. I wouldn’t advise to do so, simply because IMHO no game should be installed to C:\, but that’s just a habit of mine. Try this:
mount c /storage/sdcard/DOSGames/Ravager
imgmount d /storage/sdcard/DOSGames/Ravager/game.ins -t iso
If that doesn’t help, but you can access drive D: in Magic Dosbox, try a complete reinstall, so the game will get the correct path settings.