As far as I can tell org-mode doesn't support warning periods for plain timestamps (at least I didn't figure out a way for events with plain timestamps to show up in the agenda), but Orgzly does.
Feel free to close this if you don't want to support this in org-gcal directly.
This issue should make it easier to find a solution for people that still want warning periods for Orgzly either way. :)
As far as I can tell org-mode doesn't support warning periods for plain timestamps (at least I didn't figure out a way for events with plain timestamps to show up in the agenda), but Orgzly does.
I've written a function that does add a warning period in my personal config: https://github.com/fapdash/dotfiles/blob/e370d5c7633c78e072259acec0057922c660a191/emacs/.emacs#L1166-L1179
Feel free to close this if you don't want to support this in org-gcal directly. This issue should make it easier to find a solution for people that still want warning periods for Orgzly either way. :)