Expertise in the problem domain is still very helpful.Are we still discussing the gamist concern of skill in play? On surface, the expertise demanded is expertise in game system.
(I know of complexities to that, but it seems like a safe place to start.)
E.g. hanging out on r/WarCollege listening to military enthusiasts analyze past and current wars from an operational and strategic perspective is affecting the way I will approach GMing war scenarios in the future. I'll be better at modeling how they work.
Concrete example: instead of only scenario hooks like "you must (please!) destroy XYZ enemy army" and "steal the superweapon and use it against its creators", after reading about how one side S1 in a certain ongoing war seems to have developed the ability to real-time decrypt the encrypted (Motorola 256-bit encrypted tactical communications systems) drone communications of the other side S2 and thinking about how S1 must have done it (both sides are very corrupt so simply bribing or blackmailing someone in S2 for a high-level private key seems plausible)...
The GMing impact of that increased knowledge is a scenario hook where someone approaches the PCs offering to sell them information that can help their side of the war. If they pay up they learn that Famous General XYZ on their side has been sandbagging his own army's efforts because his twin children have been kidnapped and are being held hostage by the enemy. If someone rescues them, XYZ will stop being curiously ineffective and the eastern front will likely solidify.