Given the different roles of player and GM in most games, I don't think that agency means the same thing. We evaluate player agency in how their decisions and choices affect the game world... that their decisions matter and can change the state of the game for better or worse.
Essentially, it's an evaluation of their potential impact as a participant of the game.
What does that mean for the GM? Why would we expect their impact to be limited? Even in games that have more rules that bind the GM's actions, there's no doubt that what the GM does can impact play significantly.
The question would be more about why. Why are they trying to influence the game? Toward what goal? In response to what?
Players have goals in play, and agency is about how they try to achieve those goals, and their chances to do so.
What are a GM's goals in play? If they have any, I'd expect they'd be more separate of the fiction within the game world. They'd be more about "providing challenge" or "rewarding good play" or the like. Those seem very different from player goals.