I have a GTRW that was created by the insurance company and is supported my me and my Doctor however my employer doesn’t like because the GRTW has me returning to full time duties much faster than my employer would like
Can my Employer prevent me from working my increasing hours because they don’t “approve” the GRTW?

This seems wrong to me.

Typically, the employer is obligated to participate in the RTW process and if they refuse, you could have a claim for constructive dismissal. If you're non-unionized, and your employer is refusing to let you return, I suggest giving our employment team a call at 1-855-821-5900 to discuss. If you're unionized, please reach out to your union rep for assistance.