If you want to clear the Gmail cache on Android in order to fix the gmail outbox queued issue then you must follow the steps that are given below:-
1. The first step is to go to the Settings of your Android device.
2. Then click on the Apps from the menu.
3. Now tap on All Apps and scroll down to find the Gmail app.
4. Then tap on Gmail and then on Storage and Cache.
5. Now finally tap on the Clear data and Clear cache options to clear the app’s cache.