First, the GM is the final arbiter of the rules:
You also act as a final arbiter of the rules, determining the outcome of the PCs’ decisions and how that impacts the story as it unfolds.
Secondly, the though the chairman, not God, the GM is specifically called on to judge the use of the rules:
It’s also your job to make most of the moment-to-moment decisions about what’s legit and what’s not regarding the rules.
Thirdly, given the above, the GM is given several tools in his/her toolkit for resolving these actions in a way that everyone deems is fair. Some examples are below.
Specifically in the case that a skill could be seen as helpful, the SRD's section on edge-cases has this advice for the gamemaster:
When you run into this, talk it over with the group and see what everyone thinks. It’s going to end up one of three ways:
- It’s too much of a stretch. Consider creating a new skill.
- It’s not a stretch, and anyone can use the skill that way from now on under the same conditions.
- It wouldn’t be a stretch if the character had a stunt that allowed it.
In the above example of fishing using Electronics, along with the SRD examples quoted, the player's narration of this action can absolutely be influenced by the skills present on the character sheet. But the narration is what the GM receives, and it is up to the GM to frame what the difficulty of a roll is, and what success and failure look like (all from What to Do During Play, which is written to the GM).
In that case, absolutely, the GM (and ideally the players too) should be thinking in terms of the Golden Rule and its corollary the Silver Rule (same page), and attempting to re-frame the mechanics to match the narration.
If the player is describing using knowledge of the outdoors to accomplish the task, then by all means, insist that the roll is an Overcome roll using the Survival skill, for example. If the player is describing a technological feat that should improve the character's chances of success, or change the fictional reality, then this should instead be a Create an Advantage action using the Electronics skill!
The System Toolkit has a great explanation with examples for just how the Create an Advantage action might be useful in various kinds of situations. Imagine that in the above example, the character created some kind of High-Tech Fishing Gear. This aspect could change the frame of the problem, or could simply be used for as many free invokes as the player cares to keep rolling for.
After all, if the time available is unlimited, the character will eventually catch a fish, no questions asked. So why even roll, if that's the case? If, however, owing to his/her relatively poor fishing skills, the character is using precious time to build toward making a valuable catch... there's your dramatic story of a pro-active, competent but not omnipotent protagonist playing out as a consequence of the player's choices.
On Custom Skills
Fate Core's skill list is already fairly exhaustive, with pretty well explained and constrained skills by default. That said, creating and tweaking skills can be an integral part of a unique, mechanics-backed setting for a game. The Fate System Toolkit has tools to do this, but even to the most extreme "each player writes his/her own skills", there's an expectation that everyone should be on the same page about what skills can do.
In particular, the example about Fishing using the Electronics skill sounds like it could be a fundamental disagreement as to whether Electronics is a traditional Skill, or a more FAE-like Approach.
The GM, in collaboration with the players, has the opportunity to make these decisions at any time - but the sooner any misunderstandings are taken care of (like during game/character creation), the better off the game will be.