Gyms in 'Pokémon GO'

When you've reached level five, you can choose a team your side of the battle. Gyms are special places that can be captured by one of three teams: red, blue or yellow.
Often Gyms are well know public places, like: town halls or libraries. In Pokémon you'll recognize these gyms as towers. 'Free' Gyms are gray, and can be claimed simply by being the first one.

Today I was walking downtown (in Montpellier) and, in the main square, there's like a gym with a very high level. And, just as I was walking past it, it turned gray!! I was ashtonished O.o I even managed to get my highest level Pokémon in! That was the most exciting moment of the day (it's easy to beat though so it wasn't that mean xp)