It is common to gain weight, but it's not necessarily due to menopause or HRT. Basically as you age you do need to modify what you eat.
I gained a fair bit of weight over a period of about 3-4 years and I couldn't lose it no matter what I did. I thought I was doing all the right things - eating a very healthy diet, no processed food and very little sugar, but nothing worked. So of course I blamed everything (menopause, HRT, metabolism) except myself and what I was shoving in my gob.
I joined Weight Watchers last Christmas, became more aware of correct portion sizes and what I was mindlessly having between meals, and I've lost 20 pounds. So it obviously was what I was shoving in my gob.
This is what worked for me, but it might not work for you. However, I'm sure if you compared what you are eating to what you are supposed to eat, or maybe whether or not you are doing enough exercise, you might just find it is diet and exercise related.