Liquid metal is not necessary here, and generally a bad idea if you've never used it before; if it rolls off the die and contacts SMDs, and you power it on like that, you can kiss that gpu goodbye.
Or, if you have an accident and it squirts out the tube... that's gonna be a pain to clean up.
The CS107 V2 does not look to have the most favorable airflow for gpus, but if it was fine before, then:
1)Did you dust off the gpu when you took it apart?
2)You can confirm that the gpu's fans and other fans in the chassis still work?
3)MX-4 may not have been ideal here. The surface of a bare silicon die is different enough that it changes what pastes work. MX-4's formula may not be thick and sticky enough for this. Are any of the following pastes...