I'll be happy to help put your minds at ease - most gyms are not like that. I belong to a very large gym full of people, most of which are overweight. And I'll bet that many of the fittest really had to work to get there.
And I am a people watcher, and for the past 2 years I've 'looked' for injustice.

The uber-fit ones that one would think may be intimidating are only interested in their own bodies. By exercising and trying to get in shape, you have made yourself one of 'them'. Everyone is welcome, everyone has a common goal. If anything, you'll motivate other overweight people by being there also.
I was worried once, too. I thought I'd throw up before I made it in the door for a tour! If I hadn't done it, it would have been a huge mistake for me.
Good luck, and be brave.