Congrats man! Those are solid gains! I used to be huge into working out as I played football and baseball, but hurt my back from a sports injury in college and haven't really touched weights since. I had worked out from the time I was in 8th grade until my junior year in college. I've always been a hard gainer as well weighing in at 175-180lbs. It took many years of consistent lifting to have a solid base level of muscle built up.

Throughout high school, I took creatine on and off a few times a year but concluded that it was more of an inflatable state than anything remotely close to permanent. Once I got into college, I tried a few different 100% legal and what I would deem safe stacks. If you're interested, send me a PM.

But my gains were pretty ridiculous, even as a hardgainer!
I'd only really gain like 5 or 7 lbs and stay in the 175-180lb range, but my strength would go through the roof! Started maxing out at flat barbell bench at 245 and ended up at 295. Flat dumbell bench went to 110 dumbells. Incline bench went to repping 225 about 8 times. I ended power clean at 245lb reps. Those are solid numbers I'd say and I never looked anything close to an Arnold. Just looked like an in shape 180lb 20 year old.

As long as you do research on a supplement you're taking and understand its properties, I'd say they can be safe. Most of the people who told me supplements are unsafe are the ones who never stepped into the workout facility.