Welcome to HCM,
You are already taking treatment for Anemia
and your Hemoglobin
is also reasonable 12.8 gm% and can continue to take B12 injection
on monthly basis and you can start work out gradually .
My advise would be to start cardio work out initially later on weight trainig may be done.
There should not be any problem in Gym.
If there is any intake of alcohol it must be stopped completely.
Good luck to you,