They are the same cooking method. However, an outdoor gas grill is typically not built as well as indoor gas grill of the same cost, because they need to include a housing. Practically speaking, the outdoor gas grill will be exposed to more temperature and environmental variations, too.
Both grills have heat output measured in BTUs. They are directly comparable.
The indoor grill will be a source of heat which may or may not be welcome, and a source of indoor air pollution depending on incomplete combustion.