Couple of things. First, although the parties were directed to file responses to the intervention motions by a specified date, that order was directory, not mandatory, i.e., setting a filing deadline for filing. So Gore did not have to file anything, and there is no penalty because he "failed" to do so.
Second, Appellants take what I thought was an unusual approach. They opposed the motions by Brady Campaign and CPOA on the basis of lack of standing, and I rather suspect that the Court will agree.
However, the opposition to the AG's motion was a surprise. After arguing that the AG has no statutory right to intervene, Appellants conceded that the court nonetheless has discretionary authority under the rule to grant the motion. Appellants expressly stated that they did not oppose the court's exercise of discretion to grant the motion to intervene. This is kind of strange at first take, because there are plenty of good factual reasons that could be argued against granting the motion, specifically the fact that the AG has on numerous occasions declined to voluntarily participate, or has filed motions to dismiss in cases that sought to add the AG as a party.
So why did Appellants/NRA roll over on a motion that there was a good chance they would win? Some have suggested that it was to show "respect" to the court, and bow to the inevitable. I think that is nonsense. Instead I think that a strategic decision was made to allow the en banc petition to go forward in that petitions have been filed in the companion Richards v. Prieto companion case. Appellants did not want to take the risk that the Richards petition would be granted, but that because of the denial of the motion to intervene, they would not have a place at the table. And this is especially so given the long standing animosity between the NRA/Mitchell faction (Peruta) and the CalGuns/SAF/Gura (Richards) faction, plus the great potential that these cases will find their way to the Supreme Court cert docket.