Not to worry.
Your liquid cooler is about as heavy as a 120mm air cooler.
If you have room, lay the case down level so that the graphics card is vertical.
Perhaps in the trunk.
That way, there will be no torque on the card if the card moves.
Pack around the case so that a sudden stop will not move the case.
If you are really paranoid, orient the card to the direction you are traveling so that a sudden stop will not torque it. I imagine you have moved a case with graphics before and had no issues.
When you arrive, check all for anything amiss.
Perhaps first run without the case side so you can check for the possibility of a leak.