Can Confirm. I have a Sapphire Radeon RX6400 in my 9020 SFF and works great. It does get a little warm when playing games though. I average about 75-85°C and the system fans run regularly. I have it paired with a Dell E1909W monitor and can hit 75 fps (maximum allowed since said monitor is only 1440x900 at 75Hz max.)
For some of you that may have a DELL Optiplex 7020 SFF and want to upgrade the GPU, RX 6400 Low Profile with 4GB is the best solution at affordable price.  at Amazon [URL] RX6400 Low Profile 4GB [IMAGE] If you are not so sure about this, then check this YouTube information:  [URL] Also you can check this out for the Specs and type of graphic cards you can adapt: [URL]    
Graphic Card GPU For DELL Optiplex 7020 SFF Radeon RX 6400 Low Profile 4GB GDDR6