In my spare rig, g20cb, I have the stock 2tb HDD along with an 850 evo 1TB SSD in the spot where it would be factory. You’ll need the 90 degree sata cable that came with the pc and connect that and a power cable in there and setup the ssd in the bios. If wanting to run it as a boot drive, the Samsung magician software can copy everything over to the SSD no problem you can look up how to install an SSD in a g series pc on YouTube. One of the members in here has a few videos of it. It’s the welshwhoopie guy.
But in short, yes an 850 evo 1TB SSD, 2.5” version, not the M.2, will work in the g20cb. And it makes a big difference.*