Does the license of Quake 3 Gold purchased on GoG allow me to use it for various ioQuake ports ( e.g. Android ioQuake port ) to play the game on non-PC platforms? Or will it be illegal?
[ Provided I have Quake 3 Gold installed only on one machine - an Android smartphone ]

Licenses and agreements are bullshit. I would advise you to ignore them.

As for legal, well, here's the thing. Legal where? Based on law (whose interpretation of it?), based on a case? Based on rulings that are supposed to govern the greater economic region? A while ago, some assholes in EU ruled that you need a license in order to use copyrighted software because technically your PC will make copies (in RAM, caches, etc.) of the software in order to be able to execute it. At the same time, EU considers shrinkwrap licenses invalid. Does that mean you never get a license to use a software if the license is only shown to you after you open the package?

There are no easy answers, it's a bullshit system.

Here's what you agreed to when you joined GOG:

Yeah, publishers like to switch between "you bought a copy, not a licence" and "you bought a licence, not a copy" depending on what benefits them and not you as a paying consumer.

You paid for the game and you have the moral right to play it wherever you want. If they have a problem with you playing your PC copy on a Raspberry Pi, let them sue you if it pays for them.