To recover a Gmail account on Android or iPhone, there are several steps to be followed.
When your Gmail is not syncing on your Android or iPhone, you need to look for the issue that causes the glitch, like a disturbance in the internet connection, a problem in the settings, updating, or another possible glitch in the system. To fix the problem:
1. Check the Internet Connection
The first basic step to fix such problems is to check the internet connection before looking for the complex problems. To ensure that the Wi-Fi is working properly, try to open any website and check if it’s loading properly or confirm by streaming a short video. Similarly, to check the mobile data connection, you can run the same procedure. A weak internet connection, either from Wi-Fi or Mobile Data, can cause such problems. 
2. Refresh the system 
Manually refreshing the system or Gmail is required sometimes. On an Android system, you can refresh Gmail by opening Gmail and swiping down from the inbox to trigger refresh sync, while on an iPhone, you can just pull the mailbox list for refreshing faster. 
3. Check sync settings
If the refreshing does not work, then you can check the sync settings and try force syncing. On the Android system, you need to go into settings > Accounts > Google, then select the Gmail account and enable the “sync Gmail” option. On the iPhone, there are IMAP accounts, and similarly, you need to go into settings > Mail > Accounts, then select “Fetch New Data” and enable “Push” or “Fetch”, whichever option is available.  
4. Restart the Device
One other simpler way to correct a temporary glitch in the system is by rebooting or restarting the device. to do this an Android, you can hold the power button and restart the device, while on an iPhone, you can hold the power button and slide to power off and then turn the device back on.  
5. Update the Gmail App
It is very important and necessary to update the Apps from time to time for security purposes and to fix bugs, if any. You can easily update the Gmail app from the Google Play Store on an Android device and from the App Store on an iPhone. 
6. Free up Storage Space
Gmail stops syncing sometimes when the device storage is full. You can free up the space in your device by deleting the unused apps or deleting very large files that are not necessary. 
7. Enable Background App Refresh
Allow background activity for Gmail to run/sync in the background. On the Android system, open Settings > Apps > Gmail and then allow background activity in the ‘Battery’ option. iPhones have ‘General’ option in the settings where you select Background App Refresh and then enable it for Gmail. 
8. Check Google server status
Sometimes the company server is not working properly because of some updates or bug fixing. Check if it’s a server issue. Then you can wait until the server is working again, which usually takes a few hours.
9. Clear Gmail App Cache for Android only
There is some cache data that blocks the syncing of Gmail, which should be cleared regularly. You can go to Settings and then open Apps, select Gmail, go to the storage, and select “Clear Cache”. 
10. Remove and Re-add Your Gmail Account
Removing and re-adding the Gmail account to your device creates fresh connections, deleting all the previous glitches. This could be done by opening Settings > Accounts > Google and then “remove account”. Then you can add your account again.