There's really nothing that we could accurately compare it to, since they've never actually played a standalone show in the US before and the entire...set-up, let's call it, is completely different in Japan.
For example, even though in Japan they have standing room-only venues like the HOB, every ticket has a specific number and you must enter the venue in sequential order. The goods/merch tables open up a few hours before doors and are located in a special area near the entrance of the venue. No GA venues in LA really do things like that, although I suppose there could be a chance that they will be offering goods ahead of time. I do remember at one of the shows I went to at the HOB Sunset Strip in the past that a band did set up a merch table in the parking lot entrance before doors.
It might be a good idea to just wait for more details about the M&G first and then maybe plan to arrive an hour or so before the time it'll start.