BTW, Gemini Rue only got released in iOS because it was totally reprogrammed from the ground up - nothing to do with AGS itself really, although the open sourcing obviously helped!
This is wrong. Gemini Rue uses the iOS port of the AGS engine; it wasn't remade from scratch. There are some details about changes made for tablets in this interview. Many of these things would be done on the game level, not the engine level. And Janet Gilbert says:
The engine-level technical challenges were mostly handled by the AGS porting team ? so a big thank-you to everyone on that team, especially JJS (Jochen Schleu) who fixed so many tricky bugs in the engine for me!
A lot of games work well in the Linux and Mac ports already. Obviously it's more work to adapt a game for mobile platforms (like making the hotspots larger or reworking the combat in GR for tablets).