Turn off the target app's Setting
In the case of Gmail
1.Tap the 'Settings' and your gmail account from the menu.
2.Tap the 'Manage notifications'.
3.Tap the 'Behaver' and select 'Make sound and pop on screen' or 'Make sound'.
4.Tap the 'Behaver'Advanced'.
5.Tap the 'Sound' and select 'None'.
6.Turn OFF the check Vibrate.
7.Turn OFF the check Blink light.
8.Tap the 'Inbox notifications' and Turn ON the check Notify for every message.
To specify the sound for each label notification in Oreo.
Please specify a label name as a keyword.
If the label name is not displayed on the notification bar, check the gmail setting.
"Manage labels" > Select label to be notified > "Sync messages" > Select other than None.
and "Label notifications" > "Notify for every message".
By the way, please check "Skip the Inbox" in the setting of the filter when the notifications overlap.