Can I use a mobile phone charger to charge Amazfit GTR?

Yes. It is recommended to use a brand portable power source as the charger, the charging voltage of which is DC 5.0 V.

Remarks: There may be an issue with the portable power sources, in that they can't fully charge the watch because most of them has an automatic shut-down function at full energy. the input current into the mobile phones becomes very little when the phones are about to be fully charged, and the portable power sources will shut down the charging process to save energy, so if the watch is about to be fully charged and the current is too little to be detected, the portable power sources may cut off the current as they interpret that the watch has been fully charged.