can I get reasonably gps signal if I mount the gps antenna for my auto meter speedo under the dash. Does the antenna really need to be mounted on the top side for a clear view to the satellites. Don’t want to drill more holes on the fiberglass, if I can help it. Thanks.

I've had good results with the antenna under at least one layer of fiberglass.

All you can really do is test. Some work even under some fiberglass/plastic. some not as much.

Unfortunately the answer is just depends...there's too many variables here. The thickness of the fiberglass...is there core material, is it soaked in water. You mention you want to put it under the dash...will you have lots of wiring around or above the antenna. GPS is line of sight so the more that blocks the signal the worse it will be. Metals are the worst. Its kinda like wifi in your house.....it will go through 4 or 5 walls no problem but you may or may not get signal through walls 7 or 8.
Ive seen plenty of GPS antennas mounted inside a boat.....but usually its inside the cabin area on a race boat where its bare glass and nothing else around to interfere with the signal. All you can do is try and see how it works. They do have some small antennas now a days you can hide fairly well if you have to drill.