Unfortunately, if you want to work at a certain gym then you will need to meet their requirements. If your certification is not what they are looking for and you really want to work for them, you will need to get certified with one they can accept. Owning a business is a great experience to have and it will help you with your future development as a trainer and give you the tools to rise to the top where ever you choose to work at. It could also help you in the hiring process.
Think about it as if you where the one who is looking to hire someone to work for you. I’m sure you would have certain standards in place which your potential employees will need to meet if they were going to work for you.