The steam store page of Tropico 6 states that the minimum amount of RAM of the graphics card should be 2GB.
However I was surprised that my GTX750 with only 1GB of RAM managed to play the game quite smoothly. Do note I had to turn the settings down to an average of medium to high, and accept the fair resolution of 1600x900.
On these settings I achieved a comfortable 50 - 60 fps, which is pleasing enough if you don't want to upgrade your card and just enjoy the game. Lowering the resolution noticeably improved the frame rate, but the final image becomes blurry, even with full anti-aliasing.
The limiting factor of playing on a 1GB card is the resolution.
On GTX750 1GB: