The LG B7 does very bad in image retention tests. Rtings.com went into a lot of depth of the stress tests they performed.
“As mention on for the C7, if you find out that your TV has some image retention after playing video games for a long time or after having watched a news channel with a static logo, there is a function in the ‘Picture settings’ page, under ‘OLED Panel Settings’ named ‘Pixel Refresher’ that will ‘recalibrate’ the screen to get rid of any imprinted images that may still visible. This procedure lasts around one hour, and the TV needs to be shut off for it to work. This can usually take care of any image retention if watching normal content (without static image) did not make it go away.”
As for image burn in - https://www.rtings.com/tv/learn/real-life-oled-burn-in-test <- This is an extensive 7 month test of permanent burn in on an LG C7. It includes weekly pictures of exactly what the burn in looks like with all background colors for 28 weeks straight.
I don’t recommend this LG line up for gaming. For starters image retention is an issue, it always has been for OLED. You can get a TCL 6 series 55" TV which performs equally as well for gaming (actually shaves a few milliseconds off input delay) and will cost half the price of this refurbished for brand new.