You should not focus on the TEMPERATURE of your GPU. That video card has its own cooling system and it is already programmed to adjust its own fans to keep the GPU chip temperature where its maker thinks is best. The programming involved will run the video card fans at a certain minimum setting - it will NOT stop them or run them so slow they might stall - at low workloads like idle, and will only speed up the fans as workload increases to a temperature the maker says needs more cooling. The REAL point of interest is what the GPU chip temp gets up to at very high workloads - like, sustained gaming in a high-demand game. As long as the card can keep the GPU under its MAX temperature rating (and that can be quite high for current GPU chips) at heavy workloads and without reaching max speed of its fans, you are getting ALL the cooling that system needs. You do not need more.
More importantly, if the video card's fan system is actually adequate for the job, doing what you propose will NOT cool it down significantly. IF the video card is provided with much more cool air inside your case, the card's fan control system will just slow down its fans to try to reach the SAME temperatures as you are getting now.