Can I use a portable power bank to charge Amazfit GTS 3?

Yes. It is recommended to use a portable power bank from a well-known brand to charge the device under DC 5.0 V.
Note: There may be an issue with certain portable power banks, in that they can't fully charge the watch because most of them has an automatic shut-down function at full energy. This function helps to conserve energy as it stops the charging process when mobile phones throttle the charging current when they are about to be fully charged. So if the watch is about to be fully charged and the current is reduced, the portable power bank may cut off the current as they interpret that the watch has been fully charged.