thats a good bit of weight to gain. you sound skinny, like me. you dont have to worry about definition, because all you need is size. i wont say that you COULDNT get 20 or 30 lbs with a bowflex, but its gonna be really hard. im 135lbs. i got a bowflex. i trained my ass off. the bowflex doesnt train the full range of motion, so the muscle CAN'T get a full workout. i saw results, but they sucked for my time and money. i do the same amount of time and work with free weights- i see results that the bowflex missed. the squat pullies on that thing are a joke. you lose resistance at the bottom- which is where your muscles need it to grow! same thing for the bench. you lose resistance at the bottom where the stretch is.
like i said before, the bowflex will give you a workout. i just dont want to see you waste your money like i did. get some free weights and use those. i ended up buying a squat/ bench rack, full weights, and dumbells after i got my bowflex. i just needed the freeweights to progress, and i wasted my money and time on the bowflex.
remember the people on TV didnt use the bowflex to get the results. they used freeweights