You haven't provided much information to go on.

Does the app report any error messages? Have you taken any steps to fault find the issue? Are you able to receive any messages when manually synchronised? Are you able to successfully send an email? Are you connecting by imap or pop3?

The first test is to confirm the app is connecting successfully to the gmail server. Navigate to settings > account > ...

1. fetching mail > incoming server.
2. sending mail > outgoing server.

Tap next on each screen and confirm the server connection succeeds.

Under settings > account > fetching mail Confirm folder poll frequency = 15 min

Check sync is enabled on inbox and other imap folders.

1. Open the account list
2. Tap account
3. Tap manage folders (bottom of list)
4. Tap folder name (e.g inbox)
5. Check "enable sync" = on

Repeat for each folder as applicable.

I'd further recommend checking the app is excluded from any Android battery optimisation and allowed to run unrestricted in the background.

You can usually access this by long pressing the app launcher icon, then tapping battery usage. The location of android battery settings can vary by phone manufacturer & Android version.