One more thing is that I have the both biasing resistors on the PCB board and the gm tube is then connected by a foot long wire and kept far. If as per suggested I can take the anode resistor to GM tube assembly close to the GM tube anode and from return wire of cathode i take cathode wiring connection as recommended by you and hence using a compensating capacitor on board, then can the problem be mitigated ?
I have R anode=3.3M
R drop at cathode =330K
Capacitance of tube = 8pF
then formula 3.3Mx8pF=Ccx330K
Cc =80pF
So if i do such that I change the wiring to cathode signal type and use 80pF compensation capacitor.