there are no restrictions on the height a gas pipe can/must be fitted to an external wall, there are installation requirements that have to be complied with ie type and spacing for clips, but nothing on the height, however like everything else after the event someone will always say " i wouldn't have fitted there/like that" it is unrealistic to protect it from every scenario although care should be taken with design of install, but if the gas pipe was damaged by the builders removing roof tiles then i would suggest the builder should have taken reasonable steps to protect the pipe, (would you rip wall tiles off without a dust sheet in the bath)
Cheers the buiders lack of care has been a real issue through this project, they managed to hit all 3 services the 1st week alone