ISO 640? Or ISO 6400?
I ask, because this photo is really really noisy. Too noisy, IMO. Which means too high of an ISO. Even 640 might be high though on a 60D body. Especially if you didn't shoot RAW.
This also looks pretty significantly oversharpened and fairly extremely cropped.
Added together, I get an overwhelming sensation of "pushed way beyond what it was in post-processing" vibe from this image. If it were a super rare bid, I would understand that, but for a gull, I don't think you can pull it off. Gulls are all over the place, and you should be able to get one flying near enough to you to you to not have to crop so much, and in a better lit situation to not have so much noise, and with nailed enough focus (or maybe that's also just the cropping amount) to not have to sharpen as much.
In other words, common bids that aren't afraid of people = much much higher expectations for technically perfect photos.
A better lens is one way to solve some of those issues. Better technique/knowledge of the species might be another (to be able to anticipate when they will take wing and where, to set your shot up). But one particularly easy way that doesn't require any practice is to shoot at a different time of day. I don't think the sunset lighting is doing a whole lot to make this picture better, so I would therefore suggest to not worry about shooting at sunset, and instead capitalize on the brighter light of more midday to allow yourself a lower ISO and a higher %age of in-focus shots to choose from so as to not have to rely on one from so far away.