Yes, definitely doable. I would suggest getting involved with the custom pinball community and learning how to fabricate your own playfields and parts. I've built an EM game from 'scratch' (ok, I reused mechs from existing games, converted to my use). That's essentially what you'd be doing, aside from the playfield and certain parts. But you've got a huge head start in that the paperwork for Gemini exists and you would not be creating it from whole cloth. Also the IPDB has lots of pics of the mechs in the backbox so you can see what you need, from a basic perspective.
You will spend a LOT of quality time with a schematic, and I would suggest that if you do not use the standard Gottlieb color scheme, you make sure you note your copy and redraw the schematic at the end. Some have done this by adding features to bingos. Now that's a challenge!
Playfield artwork and backglasses can be printed, but you'll need to supply the art. That's the hardest part (to me).