I've used Fibreglass tanks since the 60's - and still do.
Just "dropping" the bike [ off the stand say ] onto the tank will cause NO structural damage. We've had our Cobra [ replica ] race cars hit Armco at 100 mph plus and come away better than a steel / alloy bodied car.
The only real chance of damage is if you slide down the road with the tank in contact with the tarmac - and even then you are unlikely to puncture / rub away the GRP.
In the UK Fibreglass was banned as a fuel tank material for a few years, until a study was done [ as opposed to a knee jerk reaction ] to prove that GRP was indeed stronger in most situations than steel - and certainly alloy.
An alloy tank will certainly be lighter than GRP in this application as the tank would certainly be thicker than say a GRP seat.
Of more concern would be to ensure that your tank is made from a material that wll withstand the attack of Ethanol fuel - most modern manufacturers have that one sussed.
Gordon Murray [ McLaren designer ] designed and bullt a speciality [ Kit ] car based on Mini components - the monocoque was entirely fibreglass !
An alloy tank would certainly be susceptible to a puncture type contact, whereas a GRP tank would not [ ex. CRASH HELMETS ! ]
Attached pic is of one of my Porsche 356 Speedster replicas after the customer was hit by a GM Astra van at 40 mph - the Astra had to be towed away !