Is it okay to gain weight just as long as im healthy?

No: very risky!: Gaining weight in your 20s usually means you have a lousy diet (fast food, or too much sugar, fat, and red meat) and don't exercise enough. Check out "healthy eating" at the harvard school of public health web site. It's much easier to gain weight than to lose it, even more so as you get older. These days lots of healthy 20-year-olds become diabetic from poor diet and obesity.

Depends: Depends on how much weight. If you want to stay healthy you need to maintain a body mass index between 18 and 25. You can find bmi calculators all over the internet.