so cool! congrats!
i defo think you can use it - just maybe before or after your allocated work hours etc. I wouldn't recommend going during lunch etc because that's normally when they expect you to be doing coffee chats or engaging with other participants. But then again, if you're going before of after work, it might just be easier and more convenient to use an external gym!
also I really need to get glasses or smth cos I read that as "Ive been offered a scheme at an in-house gym". I was about to congratulate you on that achievement before I realised something was wrong