Question about a weight gaining supplement.?

Hey guys, i've had a weight training routine for about 3 months now, i've definitely seen results but i'm definitely not where i would like, or should be, for the amount of work i've put into it. I'm definitely doing the exercises correctly, but my main problem is my eating habits outside the gym. I eat very healthy, and i've been drinking and eating a lot of protein for muscle growth but because of my job, i don't have the time to be eating every 3 to 4 hours as i should be. I recently bought a weight gaining supplement and i'm just wondering how i should go about with this. I want it to help with the muscle building but i definitely do not want to gain fat. Does this normally happen (fat)? SHould i worry about doing more cardio while drinking this? Also, how should i take this in combination to my current protein powder, just drink one and not the other, or change them through out the day?

Another thing, i have a VERY fast metabolism, which could be another important factor.

Thank for the answers!