Use those tricks to get a purple beam raid and summon G-Max Pokemon:
1. Get a purple beam raid via Wishing Piece:
(1) Set text to slow (only until you get used to it then set text to normal)
(2) Turn off auto-save
(3) Save the game when you nearby a den
(4) Say yes to throwing a wishing piece and saving
(5) If light is not purple after saying yes, immediately go to home menu, restart game, and try again.
2. Summon an G-Max Pokemon (If light is purple):
(1) Let the game save as usual
(2) Make sure you are not connected to internet via y-comm
(3) Turn off syncing time via internet in system settings
(4) Go into the raid and press invite others
(5) Once you see the timer, go back to home menu and set the date forward by one day
(6) Go back in game and leave the raid screen
(7) Interact with raid and get 2000 watts and potentially new Pokemon
(8) If new Pokemon is not an G-Max Pokemon, try again from (4) to (7).
Den Locations and Pokemon Listings (common = red beam, rare = purple beam):
https://www.serebii.net/swordshield/maxraidbattledens.shtml