I have a Losi Tenacity TT Pro with a spektrum 130 esc.
I wanna switch to a different brand of plug for the battery.
does the gray wire just land on the post with the black wire?

If you are talking about the small wire in between the 2 thick power wires. That is the smart communication.

Yes.
I just installed my new connectors and used a piece of shrink tube and folded the gray wire back.

If you do not use smart technology (like a smart charger or telemetry) then yes, that wire is not needed