There are many ways to approach this, but you have an advantage actually being at a gym. Try gradually building a rapport with people there, find out what their goals are, and then discuss how SS can help them meet those goals. If you show you are invested in the clients even though you aren't actively training them and it will go a long way. The other thing to do is get your friends and family in and train them. You will need as much experience coaching as you can get so that you can retain actual clients down the road and you can use the sessions with your friends and family to get some of this experience but also to generate promotional material (photos, videos) to advertise on social media.