If the first place that the gas pipe is meaningfully accessible is after the solid floor (and before connection to something or a branch to more somethings such as boiler, cooker, etc,) that would be a reasonable place to bond.
AFAIK all internal gas pipe work is metal, not necessarily designed for good electrical bonds, but almost certainly a decent conductor so a bond there should do the job of ensuring the gas appliances remain at the same potential as the house CPC system and you don't roast the boiler's 0.75mm CPC under open-PEN cases, etc, etc.