There are several good companies for fiberglass pools. I would go with Blue Hawaiian Fiberglass Pools.
One of the differences between plaster and fiberglass pools is that fiberglass stays smooth. Plaster can become rough. Another difference is that unlike plaster, fiberglass does not chip, and crack.
Fiberglass pools are more durable than concrete pools. Concrete pools tend to require monthly cleaning to prevent algae from growing on the walls. Fiberglass pools tend to need maintenance roughly every 10 years.
Fiberglass pools are less susceptible to having problems with the finish if your water chemistry is off. They do tend to have some problems with warping that you won't have with a cement pool. The fiberglass liner is usually cheaper also.
Yes it seems that whirlpool does make fiberglass swimming pools but I could not find an actual fiberglass swimming pool from the keyword listed by whirlpool.
I am a big fan of Fiberglass pools. I have had vinyl liner pools and concrete pools. Fiberglass pools are easy to clean, they manitain chemical balance better, and the surface does not need to be refinished as often as concrete pools. The only downside is that the surface of fiberglass pools can be a bit more slippery than concrete but that has not caused any problems for us.
There are many sources online that have information regarding the installation of fiberglass pools. Among these is YouTube which hosts a variety of videos about installing almost any kind of fiberglass pool.
Here is a company that specializes in building inground fiberglass pools and spas: http://www.vikingpools.net/
gunite is a type of sprayed sand cement used for the bottoms and sides of pools then fiberglass panels are fitted in to form the complete side to the top Fiberglass pools are also produced as complete pools and shipped to site by truck. the obvious difference here is that one pool is made from reinforced cement "Gunite", reinforced concrete, "shotcrete" and reinforced fiberglass and all kinds of systems in between.
No, they are usually in-ground. Many prefer fiberglass for it's lower cost, easier maintenance, and better algae control.