You run into the office, find the gun, and run the hell out of there without incident. A couple of other employees give you funny looks as you run like hell, but soon none of that will matter.
As soon as Dr. Marks started telling you about this amazing gun, you had one thought: giants. Ever since you were little you've been fascinated by giants, and the thought of what it would actually be like if there was a giant walking down the street in real life, tearing stuff up and doing whatever it felt like.
Now you finally have the chance to make that dream a reality. Sure, it's probably dangerous, but who cares? This is the one chance you've got. Turning the switch to "grow," you look around and wonder what kind of giant you want to see. You've always wanted to see what a real-life giantess would actually be like, so you could find a random good-looking girl, and enlarge her to massive proportions before she knew what hit her. Of course, if you wanted mayhem, maybe you should enlarge a whole group of people and see all the different things they decide to do.
But you also have a female friend or two who you think wouldn't mind becoming a giantess, and that way you could ride on her shoulder or in her ear and make some suggestions on what she should do. That would be a lot of fun. Finally, there's always the idea of becoming a giant yourself. You wouldn't mind wreaking havoc for a little while.