A follow-up question was posed (privately), about my use of “do not affect the match score”. I apologize, I sometimes use English very casually, and that can confuse my meaning for non-native English speakers.

The example and question asked:
After game one, a midround deck check awards a Game Loss to each player. Is the match over, recorded 1-0?

In that example, the match continues until one player has won two games (or time expires).

To clarify, simultaneous Game Loss penalties are treated as if they never happened, and the match continues as per usual - but the penalties are still recorded as Game Loss in the reporting software.

In the very unlikely corner case where both players receive, simultaneously, Game Loss penalty during a game, just let that game and match continue (and record the penalties). This is unlikely, but if both players have already received multiple Warnings for GRV, then commit a double-GRV, it's possible…

d:^D

Edit: I've been informed by the MagicFest scorekeepers that they are able to assess Match Losses for both players; this has already been necessary for double no-shows, fairly common at GPs last year. Please use that, instead of undoing the match (as illustrated by Francesco's post); undoing the match is a useful trick when various other things go wrong and you need to re-pair just a couple matches.