if you are at college why did you have to organize a get-together or pub crawl. The idea is to not hang out with class members but get a feel of what else is out there.
Show up to a hot spot and mingle not invite BIO4011 to drinks after lab.
It's actually very common for grad programs (at least based on my personal experience with clinical psych programs at various universities) to organize/plan these kinds of things. You spend so much time seeing the same people that you really don't have much opportunity to socialize outside your area. I've been fortunate in that I have a good number of non-psych friends, although they're grad students in their own rights (math and engineering mostly).
Also, you typically avoid hanging out with undergrads, as at some point you're likely going to be their instructor; it's more difficult to maintain appropriate professional boundaries if your students have seen you quasi-intoxicated while out at a bar. When spending time with fellow grad students, you don't have to worry about those sorts of things. You also happen to generally have quite a bit in common.
Although to the OP, sadly, you'll just have to get used to the gossiping. Whether it's about your, their classes, their "super-insane work load" (or lack thereof), or their major professor, people in grad programs just seem to like to talk. Your best bet is to simply remain friendly, ignore the rumors, and go about your business.
