Make & Model of your GPU Card?
When the fan doesn't turn on does the GPU card overheat?
If your GPU card is overheating then I suggest you make sure both GPU Power cables are correctly connected.
That the fans are spinning easily and silently without any resistance.
But if the fans turns on after the GPU starts to get hot, over 60c or hotter, then it is working normally.
Zero RPM is a feature that when the temperature of the GPU card reaches a certain temperature (~55c -60) it will automatically start spinning.
In Radeon Settings you should be able to make an Fan curve profile to have the GPU Fans turn on at an lower temperature and higher speed.
NOTE: When you first power on your PC, when BIOS checks you GPU card the Fans should turn on for a second or two.