If you were able to use the Gmail app on your iPhone XR before it abruptly stopped working, it's most likely just a random bug with the program. To clear this up, use these steps to force close the Gmail app on your iPhone XR:

1. Swipe your finger up from the Home screen and then pause.
2. To move through the app preview, swipe right or left.
3. Then slide up to close the Gmail app preview. Carry on with the remainder of the app previews on the screen.

Clearing all background apps can help prevent other suspended programs from interfering with iPhone features and apps, especially the Gmail app. Following these methods, reboot/soft reset your iPhone XR after deleting all background programs and Gmail:

1. For a few seconds, press and hold the Side/Power and Volume buttons at the same time.

2. When the Slide to Power Off instruction displays, release the buttons and move the power off slider to the right.

3. Press and hold the Power/Side button for another 30 seconds until the phone reboots.

A soft reset or reboot of the iPhone might assist clean away any faulty files and temporary data that have been saved in the phone's memory as cache. It also refreshes the phone's operating system and applications, making them run faster and smoother. This has no effect on any saved data on the phone and hence will not result in permanent data loss.