Have you seen a dietician since you were inpatient? Often meal plans get adjusted over time as your body starts to get used to eating again. If you haven't seen one recently, it might be worth seeing one to check in and see if it's just your perception of what you are eating versus actual reality.
That said, I know many folks in eating disorder treatment who were overweight to start with that did gain weight once they started eating regularly and following a meal plan. It happens. Have you looked up information on the health at every size (HAES) model? It might give you some ideas for ways to challenge some of your thoughts.
Gaining weight and dealing with your body changing can be really, really hard to deal with, even if it was necessary. I know it's been hard even for me to find providers who are able to validate that those changes are still distressing. I found by stepping outside of the eating disorder providers bubble I was able to get providers to look at things from a different perspective and that was helpful for me. Do you have any providers like a therapist or someone you can see besides your psychiatrist and GP?
I think ed practitioners are really good at helping you stabilize behaviors and get to a healthier place physically, but they're not always so good at dealing with the mental stuff. Granted you need to be healthy and eating to deal with the mental stuff, so if that is a place where you are now, it might be that this is the first time you are having to try to look at and tackle stuff that being physically unwell from an ed was masking. That said, really well done on coming so far in recovery!
Is your GP and/or psychiatrist tracking your weight? Do you have releases in place that they could talk to one another? If your psychiatrist does not think what is going on is due to your meds, maybe they can advocate to your GP to look at other reasons that might be causing weight gain. Similarly, it doesn't sound as though you tracking your weight is doing you a lot of good? What do you think it would be like to stop weighing yourself and/or have your weights by your GP done blind?