It's quite common for neighbours' services to run within a property. The important thing is to have your solicitor draw up an "easement" which will be noted on your Land Registry records. Whether you decide to charge for this is entirely up to you.
As I've said, this is quite normal. No prospective buyer with any sense would ever baulk at this.
There are two ways the Gas Contractors would go about it........
Either ... dig a trench which would be reinstated at their (or the neighbour's) expense. Make sure this is agreed.
Or .......... usually most likely ........... they will "mole" underground without disturbing the surface.
So... talk to your Solicitor (at your neighbour's expense), and talk to the Gas Contractors and get a plan of action from them.