When it comes the gym (or any fitness community), the idea of "belonging" presents a double entendre.
In one case, there is the act of belonging. This might be as simple as paying a membership fee or registering for a certain number of classes.
Then there is the feeling of belonging. From your perspective, this sense of belonging is far more significant.
Many gym owners and fitness community leaders go to great lengths to welcome and continually appreciate their clients. In essence, to set the stage for your feeling of belonging. This is absolutely good and right for business and just human goodness.
You thinking "I don't belong (t)here" at any point along your current or prospective exercise process is the other side of the street.
If that statement is true, it is only because the particular exercise modality in a given facility might be inappropriate for your goals or current physical condition (eg. CrossFit may not be the best choice to help you with a low back injury).
The version of it where you tell yourself that you are "other" from the "fit crowd" because you are too (insert derisive adjective here) is a lie that you learned somewhere along your way to wherever you are now.
Are you willing to let that go?