#1 These parts have no gel coat. The parts are compression molded like the agitator in a washing machine. They come from the mold with a smooth finish, but not the glossy finish that you get from gel coat in a fiberglass layup.
#2 The proper preparation would have been a wet sanding with 600 wet or dry, followed by repair of the chips and dings with filler, followed by a light colored sanding primer which is wet sanded again with 600 wet or dry.
It would be interesting to see how high the surface is beneath the labels on the tail cowl. That would give an idea of how brutal they were. Asking to have plastic parts blasted with any media is a mistake, and IMO it would be hard to get compensation for something you asked the blasters to do.
The good news here, if there is any to be had, is that sanding the primer is easier than the original paint and in the end will be about the same amount of work(possibly less) as the thorough wet sanding and filling of the chips in the old paint. You will also have a paint job that has no residuals of the previous color and a very well prepared surface.
Take the parts to the paint shop and get an estimate of what it would cost to shoot the epoxy primer for you. Tell them you're in no hurry and you are willing to wait until they are doing another primer job if they can give you a better price. Then see if the blaster will cover some or all of the cost. They should have at least warned you of what you were getting into. I suspect you will have to write this off as a learning experience.
If you do the sanding you will eliminate a large chunk of the labor in this job. When I did my K100RS i did the prep, including masking so all the painter did was the base and clear. Total cost was $500 for a complete RS fairing. Add $150 for the primer, and that would be a good price for the job. Check a couple shops to see who can give you the best price. Another way to get the cost into a reasonable range is to not use a BMW color. Some of them are special three coat finishes that will require you to go to a shop that uses BASF Glasurit paint. That will drastically limit the places you can have do your job. Pick a nice common automobile color and go with that.