"The gulls coiled over the gathering" - how unusual is this sentence?

It's very poetic or literary to describe a flock of seagulls that way - so yes if being used in those contexts. Understandable but a little odd, if being used while chatting about how seagulls stole your french fries at the beach last week.

It’s fine. Would it sound weird? Perhaps. A little bit of weird is good in writing; it made you think.

It sounds pretty weird to me. It would probably be *understandable*, if you were doing it in some kind of deliberately weird "things are not right here" piece of fiction, but 99.5% of the time, the word you should use for what birds do is "circled".

If I read this sentence, I don’t think I’d even understand what the author was trying to say.

It sounds weird. "Coiled" is used for a long, generally thin thing (like a tail, or a snake, or the wire that comprises a spring) formed into circles. Multiple snakes could be coiled up but you can't have multiple things forming a coil, unless they're tied together somehow. It also doesn't imply movement the way circling does.

It'd sound very weird in conversation, but you could potentially use it in a piece of literary writing. It'd still sound odd, but if that was the effect you were aiming for, it could work 

I don't think I would use it in a conversation because I don't think most people would understand that meaning. Certainly if you are writing something and there are other creative flourishes it would make sense, but in conversation one would simply say "The gulls circled over..."

I don't think it works, even poetically. I think of coiling as something wrapping around itself or something else, not just moving in a circle.

Coiled doesn't seem right to me. I would have said converged on or surrounded, but neither of those have the same poetic vibe

To me 'to coil' doesn't really describe the path they're taking through the sky, but rather the shape, so it sounds like each bird is making itself into the shape of a coil, or perhaps are getting ready to pounce or something, rather than flying in spirals. If I specifically wanted to use "coiled" in that context I might say the flock rather than the gulls, so I imagine the shape that the group of birds are making in the air collectively, rather than each bird individually. I think there is an argument to be made for style, I've certainly read books and poetry where I've had to take a sec to think what the author is trying to say by using a certain word in an unusual context, but here I don't think it's the best way to communicate that specific image.

It's fine. We often use descriptive terms that aren't *quite* applicable, but people will understand the meaning. It's not common to describe a coil of flying birds, but it's not unnatural either. Starlings circle, so they could be coiling around a cloud or chimney. Or swirling, spiralling, looping, twisting, bending, curving, winding... or even snaking or serpentine, like a snake coiled up. Metaphors and analogies are an important part of English. It doesn't need to be a "well-known phrase", like an idiom. Make up your own. Now, I'm going for a spicy night out, with oceans of beer.

It more means that they arranged themselves in a tight spiral. It isn't typically used though. Most people would say spiraled or even swirled if that's what they meant. I wouldn't use it myself.

It's a piece of literary language that's presumably being used to convey a sense of menace. It's unusual and you would need a very advanced sense of the language to use it without sounding weird. Even then, a lifetime might never give you the opportunity.