What is USB gong?
If the low battery symbol is flashing (when the phone is not turned on), that mean the battery voltage is very low and needs to be charged more.
So you are saying that the power from the charging cable is not going to the battery as the percentage just keeps dropping / doesn't charge up right?
I just fixed a iPhone 5s with the exact same symptoms. I ended up having to replace the charging port (It's a ribbon cable assembly that also comes with the bottom microphone and antenna cable on it.)
Better to try replacing the charging dock first, then if that doesn't work I suggest trying another battery.
The last resort is to get the U2 tristar charging IC replaced professionally by someone that does board level iPhone repairs.
Guide to replace charging port / dock:
iPhone 6 Lightning Connector Assembly Replacement
Where to get the charging port / dock:
iPhone 6 Lightning Connector and Headphone Jack
Guide to replace battery:
iPhone 6 배터리 교체하기
Where to get battery:
iPhone 6 Battery