How can one change the timezone of Gmail (Android application)?

Edited with inputs from Irfan Latif (thanks) in comments

You can change the system-wide time and/or timezone in Android but you cannot change either of both in individual apps because apps usually make use of system APIs to get time/timezone.

See Time Zone from Android Developers documentation

Typically, you get a TimeZone using getDefault which creates a TimeZone based on the time zone where the program is running. For example, for a program running in Japan, getDefault creates a TimeZone object based on Japanese Standard Time.

However it entirely depends on the developer of the app. They may use some other approach to get timezone e.g. from public IP or device location. Or they may provide an option in app settings to entirely fake the timezone. But Gmail app does not do this and uses system time.

It seems to be possible to change the time zone of Gmail on desktop by changing Google calendar time zones (as I see on net search) but that's not possible on Android.

I am not aware of any rooted method that achieves this either, though some old Xposed modules claim to work in creating dual time zone clocks.