Our advice, after hearing the same story over and over and reading TripAdvisor threads like this one, is to avoid cheap car rental websites and especially the cheapest deal on offer.
The low prices these websites quote are not enough to cover the cost of renting the car so the companies have to gouge their customers for extras. Scams include overcharging for fuel, charging to remove 'excess fuel', claims for imaginary damage, credit card charges or fines that don't exist, etc.
Gran Canaria's shady car hire operators are clever people; They have an office at the airport and operate just within the law. The only way to stop them is to boycott their services until they are forced to change.
For hassle-free car rental in Gran Canaria, book direct with a reputable company like Cicar.
More on hassle-free car rental in Gran Canaria here.