The science of body weight, nurture or genetics, that I have read from books by Drs and researcher is that about 25% of the population is just slim and they do not gain weight. The rest of the people will gain weight and have to try to keep it off. You will notice this with the people around you. Many of the studies started after WW 2 with the Marshall Plan and people who were starving and what happened to them after they had ample food. Then much more research was done with all variety of populations. People volunteer to gain a lot of weight and then told to go to their normal eating habits and some just naturally lose that weight while most do not.
First the alcohol.
Do you know how alcohol/ethanol is made?
Plant substance, yeast and sugar.
Alcohol has lots of sugar in it.
As for gaining weight. I'm at 153 and eating everything in sight. My gal has kind of blown my healthier eating. This winter I will get back to lifting, but I have been trying to get a little bulk first.