"gaming" when applied to a motherboard is just marketing hype.
Possibly a motherboard that supports dual graphics cards might legitimately be described as "gaming"
For you, the real question is if you can fit a discrete graphics card.
1. Can the card you selected physically fit?
If your case is a sff, you will likely have a problem with height.
Compare the dimensions of your new card with the available space.
2. Do you have a x16 pcie slot that a graphics card requires. From what I can see, you are ok there.
3. Is your psu adequate?
Likely not, a psu with a 8 pin cpu connector is non standard and not likely strong enough to do the job.
Here is a chart for the requirements for a variety of graphics cards:
http://www.realhardtechx.com/index_archivos/Page362.htm
The RTX2070 for example needs an 8 pin connector found on 500w power supplies.
Since your psu is non standard, you will not be able to use a standard ATX power supply.
However, if a ATX psu will fit, I think you can use a 8 pin adapter like this to make it work:
https://www.moddiy.com/products/Dell-OptiPlex-3020-PSU-Main-Power-24%2dPin-to-8%2dPin-Adapter-Cable-(30cm).html