My experience has and is in the "suck-squeeze-bang-blow" engines(in other words Diesels)
Compared to diesel engines, the gas turbine manufacturers are pretty much "in-house" when it comes to self sufficiency in component manufacturing.
Of course there are outside specialist suppliers, but compared to my industry they do pretty well.
Only about 30-40% of a diesel engine is manufactured by the brand name.
Pistons,bearings,valves, fuel injectors, governors, turbo-chargers, coolers and so-on are all sourced from specialist suppliers.