UT testing of glassfiber reinforced plastic (GFRP)

your question would get a similar response if you asked 'How long is a piece of string?' You have not defined your question clearly enough for us to give an earnest response.
Glass fibre products are manufactured in a variety of different ways. Hand laid, machine laid, copped mat spray come to mind.
The percentage of glass fibre to resin or plastic will also weigh heavily on the possibility of a confident ultrasonic test. Also how is the product cured in an oven for a slow cure or outside subject to atmospheric conditions.
We have tested glass fibre hulls of mine hunter ships for our Navy, these hulls were up to 200 mm thick. BUT because the curing was not properly controlled air bubbles that were in the glass fibre matrix did not get the possibility to be drawn out of the material. Which meant the material was riddled with 1.5 - 2.0 mm air bubbles. Which hamoered our ability to test ultrasonically.

Hope this explains somewhat the dangers of testing Glass fibre reinforced material