How would you even configure that in the TTN Gateway? And even with a GPS in the gateway (and assuming it would get a proper fix indoors), how would TTN Mapper know where the node is?
Like @sronan wrote: just set the location in TTN Console: Gateways, select yours, Settings, Location. It will take some time before the location is propagated to https://www.thethingsnetwork.org/map so maybe it takes some time for TTN Mapper to know its location too—I don’t know.