I just downloaded GOG Galaxy and started playing Myst IV Revelations in the Myst series.
1- I just wanted to know if I can play offline
2- and also what I need to do to play on my iPad, and also offline?
The iPad is about 1.5 years old, and right now I am playing on a Macbook Pro, using Mohave OS 10.14.

Yes, you can play offline. That's the entire point of GOG, and I have no idea why you'd think otherwise.

As for playing games on your iPad, I'd have no idea how to even get started in facilitating that, seeing as I don't think Wine/Crossover even exists for the platform.