Its simply plastic which is helped out strength-wise by adding fibre glass fibres. (usually glass fibre as its the cheapest, can use carbon but £££).
Not normally any lighter than plastic so front and rear bumpers prob not worth doing.
Plastic bonnet would be good but to save weight it all comes down to the design not the material.
A bonnet can be a single GFRP sheet but it'll flap and deform at speed. A bonnet needs very clever design to make it practically rigid whilst being light as poss regardless of being cf gfrp or fibreglass.
Poeple forget that cf isn't lighter than fibrglass, its just stronger in same applications. People rage on about getting a cf bonnet when a fg one can be made for fraction of the cost and if it breaks its totally repairable, where with cf you've ruined it.
GFRP can be repaired with gf so is practical. GFRP wings, doors, bonnet and tailgate would be great as long as the design thinking goes into it.