The "gang was disconnected" is a symptom which indicates one or more primary segments' worker processes abort abnormally.
The possible causes vary. EG, max_connections are reached on one segment; primary segments down due to timeout; Postgresql Processes are killed; segment server NIC issue; File system is full on segments; etc.
I suggest you open cases to GP support team with below info:
1. master logs
2. related segment logs
3. gp_segment_configuration output
4. select * from gp_configuration_history order by 1 desc;