HGH will certainly give you new muscle fibers - and you will certainly keep those muscle fibers forever. However, that does not mean you will keep big muscles forever. Let me explain using an all natural example:
A guy is a laborer for a living - he loads and unloads baggage for an airline 8 hours a day, 5 days a week, for three decades. The guy is jacked - huge monster arms, great chest, strong legs, etc. He has the body of a weightlifter because he IS a weightlifter...doing 30 to 50 pounds at a pop for hours straight. The guy gets laid off due to his airline being absorbed by another and now he is working at Home Depot. He is 50, not young, but not yet old. After 6 months at Home Depot, his arms are noticeably smaller, as are all the other major muscle groups. What happened to his muscle fibers? Did they go away? Of course not, they are 100% natural via genetics. They simply shrunk from lack of use.
Same thing will happen to the fibers gifted to you via HGH if you stop working them.
As an aside, this is not just a hypothetical example, it is my dad. He worked for TWA, which slowly started dying after the bombing of one of their planes (orchestrated by Libya) over Scotland. Somewhere I have a picture of him making a muscle when I was 10...and his arm was larger than my head.