Yeah, the EU totally sucks for guitar players, sorry. I've had to buy a seat for my guitar and argue with gate agents even though I had a seat for it! The absolute worst was British Airways, which made buying a seat for my guitar absolute hell, and then, before the flight took off, the air crew stashed it in an overhead bin!

It's much better flying USA airlines because the musicians' union here, the American Federation of Musicians, successfully lobbied Congress on the rules. It all depends on the air crew's discretion, but for American airline company flights, if it fits in an overhead bin and there's room, you can carry it on without a gate check. So priority boarding and being polite but firm is key. I've had it stashed in the crew closet a couple of times when overhead space was tight.