Discrepancy in offer vs. Actual pay, no evidence of position documentation. [NC]

I work in the service industry. My position is service captain in which I either work as a server or a host and occasionally close the restaurant for management or very occasionally come in as a closing manager. I have 2 rates, one for when I'm on the floor in a service position and within the tip pool and one when I am outside of the tip pool. I am very rarely utilized in the latter position but have been utilized moreso the last couple months.

I noticed my rate of pay for managing shifts has shown up on my checks as $5/hr less than what I originally was told. I signed for this position way back in December under a different general manager and she had me sign the offer letter and filed it. I asked my current GM about the discrepancy and she said she would check into it. She discussed it with HR and with finance and apparently there is no documentation at all that I accepted the position in the first place. I reached out to my former manager to see if she had copies of documentation. I also talked to my other manager about possible courses of action. My other manager asked what the rate I was being paid and what I was told I would be paid and when I told her she responded that it was weird to talk about payroll for her because she has nothing to do with it and also made an off the cuff remark about how she was paid less.

The next day I was called in early to talk by my GM. She reiterated that my rate was the rate that I was being paid and that it was comparable to the same position at other shops. I also was told that conversations about this needed to cease as it was bordering on unprofessional. I expressed that I was utilizing the resources I had to correct a mistake and apologized if I made anyone uncomfortable, but did not believe I was being unprofessional. Additionally, she said if the discrepancy made me uncomfortable I could relinquish my position and step down as service captain. To make matters weirder my other manager who I had spoken to the day prior seemed to be avoiding me througout shift.

I was very upset by how all of this panned out. I got in touch with my prior GM and asked her if she would be open to writing a letter verifying that she hired me for the position at the agreed upon rates. She said she would email someone on the board of directors about getting access to her work email again to see if she could pull up the offer letter.

Honestly, if they had originally offered the position at my current rate I likely would have accepted it. It's the gaslighting and lack of documentation that has me really fired up about this. I am close friends with that other manager and I am distressed that it seems she went and told my GM that she was uncomfortable with my discussion of pay with her and the fact that I am essentially being told to shut up and accept it or accept a demotion. Is there anything else I can do?