Compared to a big public university, Gonzaga may seem like a haven of sobriety. The majority of students parties, but the contingent of students who are seriously dedicated to imbibing alcohol is much smaller. Since there are no truly wild parties, no fraternities, and not enough academic or social stress to justify excessive medication, most Zags seem to be content with caffeine and alcohol. Very few students even smoke cigarettes regularly. There is very little visible drug use on campus, and students are rarely pressured to use anything other than alcohol.
That being said, Gonzaga is no Brigham Young University, either. Marijuana is not uncommon at off-campus parties, and a minority of students will have run-ins with cocaine or crystal meth. It's reasonable to expect that more students are using drugs than you think. However, the hard drug scene is neither prevalent nor noticeable. Students are at risk with alcohol—nearly every year brings serious injuries related to excessive drinking, and that doesn't factor in the all-too-frequent sexual assaults that are perpetrated in the context of drinking. Be careful what you drink, and be especially careful with whom you drink.