Welcome to HCM.
I understand your concern.
Weight gain by steroid treatment is not at all recommended. Now, which appears as weight gain without body mass increase is due to fluid retention inside your body which is a side effect of steroid treatment.
I advise you to slowly taper the dose of steroid and stop it and not to take steroids
Try to gain weight in a health way by means of good high protein balance diet along with exercise.
Hope this will be helpful.
Do ask if any further question is there.