No, you can not do that.
Not only do the pinouts differ; the tubes are of two different types.
The center tube of the ST-120 is a twin triode, nominally a 12AU7. The 6GH8A is a pentode-triode.
"Someone" was probably using the 6GH8 to replace the 7199 pentode-triode in a STOCK Dynaco Stereo 70 (a wiring change is required, the 7199 has yet another different pinout) but that amp has a totally different driver/inverter circuit from that in the ST-120.