Worked 6-10 more shifts since playing just now and still cant access gym, also tried wrestling and yoga with brother to try and open it up by working out but got nothing still
If you can't even see the option for Gym from the outside, it is most likely that you have an old version of the game downloaded.
For a period of time, I forgot to removed the v0.3 from the itch download section, so you may have mistakenly downloaded it. The latest public version is v0.4d.
Let me know if you're able to resolve the problem by doing this.