In neither case will you need to pay anything at the time of the visit by the technician to your home. The inspection fee will be charged to the corresponding supply company so it can include it on the gas bill of each resident.
You can find our inspection fees in the attached files. As you can see, the cost of the inspection is regulated and controlled by each regional government of Spain. This is why, in some regions, the cost is shared among all the supply points in service connected to the installation while, in others, it is a fixed cost per home.
If you opt for an installation company, you will need to agree the cost with them.