Fitting inset gas fire query

Can someone please indicate what exactly is involved when fitting an inset gas fire into the existing fireplace of a 1930's house.
I am removing a free standing gas fire and intend to have an inset type fitted; I already have the restrictor fitted to the floor next to the hearth but once the inset is fitted, how does the fitter connect the gas supply to the inset as I have never seen any external piping?

Also, how efficient are inset gas fires as this will be the only source of heat in the living room.

Varies depending on design, but gas is connected and outer trim, cover fitted.Heat output varies according to model, if you buy the correct model for the room size it will supply the right amount of heat.

The gas engineer is likely to re-route the pipe work so it is fitted behind the fire, so is unseen.
As for efficiency gas fires have much improved in recent times and some are rated as high 89% efficient with a decent output rating, there are some rated at 100% with much lower outputs.
Depending on the size of the room and the output of the fire it's self, plus insulation values of the wall cavity, will have bearings on the room tempreture.