I found a solution! When my S9+ on T-Mobile was upgraded to Pie, Gmail notifications worked perfectly. Then two or three days ago, my Gmail app was updated with the new Gmail look [yuck], and suddenly Gmail notifications stopped working. Here's how I just solved the problem: Open Gmail. Tap the hamburger menu and go way down the list to Settings. Rather than choosing "General settings", choose your Gmail address's settings. In the Notifications section, tap "Inbox notifications". On my phone, "Label notifications" was deselected, and "Notify for every message" was grayed out. I selected "Label notifications", and now the notifications are working once again.