Is it permissible for a government driver to use excess gasoline, beyond his needs and which he was not asked to return, to fill his private car, especially after the cancellation of the driving allowance that was previously paid to him?

It is not permissible to take the excess gasoline allocated for work, because that is an infringement upon what the driver does not own. The fact that the company does not request it does not change the ruling, unless the government entity explicitly permits drivers to benefit from the excess.