Optiplex 9020 Mt not detecting GPU. Help please!

sometimes counterintuitively it is the opposite, that you may need to turn on secure boot to let bios detect some non-Dell gpu.  not sure if this would help but give it a try.

user benchmark validation confirms RX560 is supported in 9020.  

PS user reported successful boot from RX580 in XPS 8700 (which has same Intel 4th gen cpu platform as 9020) only when secure boot is enabled.

If still does not work, try take the risk of updating to latest BIOS version stepwise (no big jump in ver.) unless already done.

I tried it and i was unsuccessful.  Any other ideas?  I didn't try the bios option because im not exactly sure how to do it.

You already mentioned that in BIOS Secure Boot is off and Legacy is on.  By "Legacy," did you mean Legacy Option ROMs?  Or is the BIOS in Legacy mode and not UEFI?

Secure Boot above.

Legacy Option ROMs above.  Legacy Option ROMs has to be off for Secure Boot to be on.  Remember to click Apply after making changes.

Re: I tried it and i was unsuccessful.

I assume you tried enable secure boot.  I believe you can only enable secure boot when boot option is UEFI, otherwise the secure boot option is greyed out.

what is your current bios version?

flashing bios has certain risk especially if you have never done it before.   

"I didn't try the bios option because im not exactly sure how to do it."

Or...what BIOS option are you referring to?

I replaced the battery and it fixed the issue!

Unreal, the one time we don't mention the battery.  Glad you got it working.  : )

Hey I have the exact same issue and what does he mean by replacing the battery? Is it the power supply?

I will answer your question for brad before he chimes in.  he meant the coin battery CR2032 aka CMOS battery.

The battery is located on the motherboard.  It is a watch style battery. It's circular.  Walmart sells them for 4 or 5 bucks. It starts with the letter C.

I'm still going to link a favorite battery since cheapy batteries last around 2 years and Duracell lasts around 5 years:  Duracell 2032 battery on Amazon  They're a little higher than they used to be with inflation, but still not a bad deal.  Single packs are also available.