I was thinking I got ripped off for a minute.
Ignoring the morality of the G2A marketplace for a moment, the historical lowest price for Starpoint Gemini 2 is the one it's currently being sold on GOG at, or $8.74. If you paid less than that, you are most likely being scammed, since why would someone sell it for less than he bought it. No, regional pricing doesn't count for gifts on GOG, the buyer pays the CN price (or his own, whichever is higher).
If it was a bonus code instead (18 characters instead of 20), it might be legit, but I don't recall any such offers recently.
Either way, enjoy your game, and hope nothing bad happens.