Possible Gunblade Job teacher?

[Zohar_Lahar]

The best candidates besides a new character:

• Cid: In theory, Garlond Ironworks would be the simplest avenue. However, I don't recall seeing Cid, Biggs, or Wedge use a gunblade, and Jessie might be a long shot. Plus, it'd be weird if they're only just now in 5.0 teaching Eorzeans how to gunblade and not earlier like when Cid first showed up in 1.xx.
• Nero: An easier choice, not only because he uses gunblades but also would be available after Praetorium. The only problem is both the Crystal Tower and Omega storylines have segments where Nero is out of commission, which would make Job quests potentially awkward.
• Shadowhunter: The good news is he's also available right after Praetorium. The bad news is it would clash with Stormblood's late reveal.

[DarthSanguine]

The best candidates besides a new character:

• Cid: In theory, Garlond Ironworks would be the simplest avenue. However, I don't recall seeing Cid, Biggs, or Wedge use a gunblade, and Jessie might be a long shot. Plus, it'd be weird if they're only just now in 5.0 teaching Eorzeans how to gunblade and not earlier like when Cid first showed up in 1.xx.
• Nero: An easier choice, not only because he uses gunblades but also would be available after Praetorium. The only problem is both the Crystal Tower and Omega storylines have segments where Nero is out of commission, which would make Job quests potentially awkward.
• Shadowhunter: The good news is he's also available right after Praetorium. The bad news is it would clash with Stormblood's late reveal.

The latter half of your 2nd point is moot, since all quests in the game canonically take place in release order. They just don't mention side content in other content if you haven't done it yet, purely to avoid both confusion (due to events being mentioned that you don't recall due to having not done the content yet) and spoilers.

[HamiltonCalder]

I kind of hope Fordola features in the gunblade class quests, in the same way Hilda featured for Machinists in Stormblood. She's the only gunblade-user we know of that isn't Gaius, and I feel like this would be a good way to give her something to do now that her role in MSQ looks to be pretty well wrapped up.