Glue on towel bars are getting pretty common. The link I have is just an example. Never used it. Basically you just need a bar with a flat backing. You can use glue or a more powerful adhesive to keep it down (Loctite makes a construction adhesive for fiberglass).
I would google "glue on towel bar" to see choices. There is tons of stuff.
As for installation, this ranges all over the place. Some come with their own install pads - where you glue down the pad and slide the backing over the pad.
Just know that the stronger you glue it down then less chance your shower enclosure will be able to be "cleaned". So if you go with a construction adhesive (99%) secure then please make sure it is where you want it for the long haul.
I know of 2 methods to create a safe mount that will support the fiberglass
Wood blocking directly behind fiberglass.
This method relies on access to the other side of the wall, to place
the blocking (1x4 is close to the spacing) such that it is against the
fiberglass and secured by 2 studs.
Special self supporting brackets (Solid Mount™)
These can be placed from inside the shower, but require you to anchor
the rear of the mounting plate to the stud (in 2 places). If the
spacing is not optimum for your towel bar, a waterproof sub mount
(spanning the 2 mounts, made of solid surface material, Swanstone or
Corian) could give you a few inches. Cantilevering any more than that
would be unsafe.
Most fiberglass enclosures don't go to the ceiling, so a "single rung" towel bar could be stud mounted just above the fiberglass.
When the shower was installed blocking could have been added to allow for a bar. Since you don't know what is behind the fiberglass to anchor the bar I would look for another method. As @HerrBag has stated even a slip that results in grabbing the bar without a fall could damage the fiberglass. In some locations an improperly mounted bar is more of a liability than no bar at all. Although not as visually appealing you may consider a rack and bar assembly that hangs on the shower head.