Gideon talks to McG and makes sure he knows the mission and what they're trying to accomplish. McG is the best flier of the lot and he needs him to determine who can A) fly well enough for the job and B) be level headed enough to not panic and freeze up and C) work well with the other fliers as a team. He doesn't want Merc to be off his game because he's worried about anyone else being in danger.
This isn't the easiest thing the Order has ever asked, and nobody should be trying it unless they feel comfortable and Merc feels comfortable with them.
Gideon is assuming Merc will be the captain on the pitch unless he decides to appoint someone else.