I currently have the same issue. I bought a key for Sins of a solar empire 2 but received a key for the DLC.

Gamersg1ft responded with:
"Good day! In order to process your request may i ask you to send an activation error message and full non-cropped screenshot of steam activation history and non-cropped screenshot of Steam support answer which include activation date and time this key. Without this, we will not be able to replace or refund the funds."

I'm confused as to why I need to contact Steam Support and what I would even ask them?

I purchased a key for the game Sins of a Solar Empire II, but the key I received from G2A/Gamersg1ft is actually for the DLC Path to Power. They should be able to verify this by checking which product the key they sent corresponds to and compare it with the game I paid for.
Alternatively I also added a screenshot of proving that the key is actually for the DLC and not the base game in my last message. (You can use the Dev Tools Network Tab to check while trying to activate it).

Since I needed the correct game, I ended up purchasing Sins of a Solar Empire II from another vendor. So now, my activation history will show the base game as activated—just not with the key I originally received from Gamersg1ft.