I like both, for different applications. Read on---
I used GelGloss for the first year or so on the Casita
. Always satisfied. Being a builder, I was accustomed to most fiberglass shower and tub manufacturers exclusively recommending GelGloss.
But then someone---I think it was sage Charles Watts---on the old c-club website recommended Meguiars Flagship. I switched and have been using that and other Meguiars "marine" products since. Tried others, never found anything better or easier to apply. I'd used Meguiars products on my cars for years---same satisfaction. No one in our area, including boat and marine stores, stocks Flagship. So I always buy it on line---last time at Amazon as I recall. (I still like to use the GelGloss liquid on the trailer bath. Why? I dunno, but it works. Usually preceded by Dow foaming bath cleaner or similar.)
My impression is that the M Flagship lasts longer on the trailer exterior than the GelGloss, considering the elements my trailer is exposed to over a season.
Back to the abrasive---I believe GelGloss has very little abrasive in it, so I don't think it would be an effective "restorer". It will "clean", which may be the result of very low-level abrasive or, probably, another agent---it has several solvents in the formula. Doubt that it has little, if any, UV protection---no claim on the label. The latter is a "plus" for Flagship.
This is not a hit on GelGloss. It's a great product. I own a bunch of apartments, some with fiberglass baths, most with formica counters. SOP on prep for new move-ins is GelGloss treatment on tubs and counters to make them look close to new. Doesn't get rid of scratches or burns, but does a great job polishing the products. (I've never ever heard of anyone rubbing through the gelcoat with GelGloss, and, personally, I wouldn't have a concern about it being abrasive.)
By the way, don't waste your money on the GelGloss spray. Useful for spot touch-up, but otherwise uneconomical. Much more mileage out of the liquid/cream can.