The game card on GOG shows as system requirements:

Minimum system requirements - Windows: XP / Vista / 7 / 8 / 10
Processor: 3 GHz quad-core CPU
Memory: 4 GB RAM
Graphics: 1024 MB graphics memory, shader model 3 (NVIDIA GeForce GTX 560 / AMD Radeon HD 6950 or better)
DirectX: Version 9.0c
Hard Drive: 2 GB available space
Additional Notes: Screen resolution 1680x1050 or higher; Dedicated video card required

But when I check the homepage of the game I get:

MINIMUM:
OS: XP, Vista, 7
Processor: 2 GHz dual-core CPU
Memory: 2 GB RAM
Graphics: 512 MB graphics memory, shader model 3 (Radeon HD 4600, GeForce 8600)
DirectX: Version 9.0c
Hard Drive: 2 GB available space
Additional Notes: Screen resolution 1280×1024; Dedicated video card required

What is listed as the minimum on GOG correlates to the recommended values on the homepage.

Is the GOG version of the game special and so much more demanding or is this an error on the game card?

The higher system requirements on GOGs game card are correct. The question has been answered here.