I'm not sure an in-universe answer is ever directly stated. If there is one, it would probably be in one of the Doctor Thunderland segments at the end of each volume. I only have volumes 14, 15, and 16 at present, but if I get the chance, I'll review the others.
That being said, I can think of a reason that the Gaichuu would be named after alcoholic beverages. Alcohol is a depressant. This means it slows down your body and brain. The wikipedia page for alcohol lists lethargy as a possible symptom for drinking alcohol. This is similar to a battle with a Gaichuu: A letter bee will fire off fragments of their heart to defeat it. The weariness they experience would be comparable to the effects depressants cause. They would both cause some loss of feeling and thinking. The wikipedia page for depressants lists one of the effects of depressants as being memory impairment, similar to how a bee could lose memories if they fire their shindan too much. Firing too many shindans and drinking too much alcohol can also both cause loss of consciousness.
So, letter bees might have seen the similarity between battling gaichuu and battling alcohol and named the gaichuu accordingly.
I'd like to point out a relevant passage from Dr. Thunderland at the end of volume 16:
To be honest, in the last volume I prudently resolved to stop sulking about my continued failure to appear in this story. But before I knew it I found myself drowning my sorrow in booze from the stress of being prudent. I'd get up in the morning and have breakfast and beer. At noon it was lunch and draft beer. My afternoon snack was a tequila. With dinner I'd have wine and sake. After dinner I'd have shochu and a whiskey nightcap. After using the bathroom I did shots of alcohol hand sanitizer...
Incidentally, I lose my memory when I drink too much. I hypothesize that the drinking is causing my brain to shrink. I wonder if it's something like What-chamacallit for Algernon. But they do say, "Ignorance is bliss." Still, my life has been a mess lately. I wonder why. Oh, right, because of the drinking... heh heh heh...
Here, Dr. Thunderland specifically mentions the memory-loss properties of alcohol. And from an out of universe perspective, Dr. Thunderland's article conveys that the author has some kind of interest in alcoholic beverages and drunkenness (the references to different kinds of alcohol are particularly interesting). I admit it might be a little presumptuous for me to assume so since drunken characters are a common gag.