either that, or going on rumble raids to one of seven maps (Silent
Forest, Windy Prairie, Eternal Tower, Bright Beach, Rocky Cave,
Fiery Furnace and Galar; which the last one can only be obtained by
spending a minimum of 5k gems at the Gem Collector via egg
purchases).
All of these above mentioned things are great for gems, and they
all have their own advantage / disadvantage.
Rumble basically gives you some Gems (e.g. around 2-4 on a 2 hour
rumble) while you have to do absolutely nothing. This is great for
a basic "income" that you can generate passively.
Note: Galar mission takes a lot longer (you can not set a time for
this) but also grants more gems + chance for nice items to sell for
PD
Sending Plushies is the method with the lowest effort, as you can
simply get Dream Points by doing 500 interactions every hour.
Especially useful when you want the shiny Plushies from the DW
shop
Royal Tunnel is the most common source for many users, as it is a
minigame where you test your Pokemon Knowledge, though I find the
outcome of Gems rather low for the fact that you have to play
actively (and invest PD to enter)