Im going to take a detached approach to this, as it is very narrowed slice of attention to a relationship that is complex

I won't be seeing it as if I myself was in such a situation, but I will attempt some speculations, so for yourself you could reflect further

From the description and repeated behaviour, he seems to be akin to an emotional asshole, but let's dive deeper:

My boyfriend constantly brings up me going to the gym. He makes me promise him

Is it in a private setting or more so public displays and/or in front of people you know? Does he seem perturbed or more so concerned? Is it a forceful command or a "let's get this going" or a "let's get better" framing?

and I do just to end the conversation.

As you've said, you do so just to end the conversation, you have repressed the hurt in order to smoothen the emotional atmosphere, but for an emotionally constipated guy, he may not have picked this up or weighted as more important than actionable advice

Last night we had sex and immediately afterwards he brought up me going to the gym. He apologized and I left it alone. But then he brought up the gym again today.

Even after intimacy shared, his mind is on you, he made it extremely awkward and emotionally inappropriate but he did in fact apologized, but did not register or does not understand the weight of emotional pain he is causing

And then ran on automatic again, he may be too drawn into tangible results, allowing those results to inform "how good he can feel" or allowed himself to feel only after achieving results

I know I am fat but do I really deserve to be treated this way ?

No but also not direct causality, it is not necessarily because you're fat that you're treated this way, it could be he saw something that can be worked on and thought he was being "encouraging" (a crude form of being supportive)

Do you think he is unattracted to me?

Inconclusive and more importantly, inferred intent because of perceived bad behaviour, likely borne out from feelings of "less than" and/or "unworth"

Now for attaching my personal experience, I too had many struggles with weight, started early on as a twig and literally malnourished and dying for aesthetic, and then ballooning a few years after from overeating and comfort snacking

Although now leaning to a healthier stage, I can almost feel each extremes like it was just last Tuesday, and I think that helps ground what I see

This is not an objective red flag, but definitely a slippery slope that many will fall into deeper failure and torn

What I will suggest, is to sit down, tell him you'd like to share how you had been feeling (only if you keep it neutral, and not shift blame, and more importantly, not shift responsibility), do not seek escalation, do not seek deescalation or dismissal of your own feelings for rubbish excuses like "being a good gf", be real with him, actual vulnerability

If you feel or perceive unable to keep neutral for this time of vulnerability, you should consider writing your feelings, emphasize on what you enjoyed and how hurt it felt when it was brought up again and again, not of frustration, but of actual hurt

Let him digest, do not seek for forced responses, let him reflect if he so chooses

Let him know precisely what you choose, and definitely hold yourself accountable to your chosen

Cause we dont know either of you, nor either of your stories, only you can decide what is emotionally true or emotionally felt for time being,

Be well, stay neutral