Hi again,
As they said when I registered her at the gym, the fact that the gym had this policy she would be reimbursed if she was to not like it.
I'd say
As they said when I registered her at the gym, the fact was that the gym had this policy that (or by which/under which) she would be reimbursed if she were not to like it. (or if she didn't like it - a more common way to say it)
Despite these corrections, I think your basic understanding of how to say this in this way is OK.
Best wishes, Clive