Try in this order :
Settings / Sound & notification / App Notifications / Gmail - treat as priority.
Settings / Battery /click the three dots / (if needed, toggle Not Optimized to All Apps) / Battery Optimization / Gmail - disable optimize battery.
IF you're having other data issues, you may have to also select Show System Apps and play around with semi - related apps n processes.
I think it's a problem with Doze, affecting numerous apps.
Good luck.