Think of the calories

The benefit being a greater amount of calories spurring growth. Sure it's not the best calories, but many people simply don't adjust their food intake for making gains. More calories on the days you work out, balanced out by a bit less on days you don't, will help you gain while not getting excessively fat. Sure it would be better if you at chicken breast and brown rice, but it's really not worth worrying about excessively.

Alcohol promotes estrogen in the body. So that's the part that's negatively effecting your gains, since testosterone and estrogen compete for the same receptors. Again not a huge deal since the key time this will matter is during your workout and for a few hours after. Having some drinks later, not a huge deal. Enjoy your life, don't try to be a machine who lives and breathes lifting weights (unless that's how you enjoy your life) or you won't stick with the workouts and THAT will absolutely kill your gains.

To sum up. If you want to be precise, it's not great for your gains overall, compared to lean protein and complex carbs. However, it's not the end of the world, and at least you went to the gym. You'll still be better off than if you didn't go to the gym and went out to drink. Try to have at least a few hours between the workout and the booze. If you decide to become a professional bodybuilder, powerlifter, or strongman reconsider drinking. Until then have fun.