If you're experiencing any visual issues, download and extract this ZIP file. Inside you will find two copies of the gg_dx7r.dll file, in two separate folders.
Locate your Giants folder (by default it's C:Program Files (x86)GOG.comGiants - Citizen Kabuto). Copy the file from the dx7 folder into the game's folder and check if the game runs properly. If not, try the file from the dx8 folder instead.
The game should run properly with one of these .dll files.