Is it reference card from NVIDIA or third party design?
Unfurtunatedly fan control on linux sucks on nvidia cards, there is no fan curves or anything, at best you can use nvidia-settings GUI thingy to set a constant fan speed.
Alternatively, you can query fan attributes present:
# nvidia-settings -q all | grep -i fan
And modify them:
# nvidia-settings -a 'GPUTargetFanSpeed=50'
On third party PCB designs (evga, asus, …), fan control is often entirely on PCB via some embedded controller along with RGB and other nonsense that sell cards nowadays.
If you can’t set fan speed manually via nvidia-settings then my guess is that this is the case, but you don’t give enough information (use nvidia bug reporting tool like suggested in top forum thread).