macOS version:
ProductName: Mac OS X
ProductVersion: 10.15.5
BuildVersion: 19F101
Kap version:
Version 3.3.2 (3.3.2.1718)
Make a screen recording
Green bar is added to the gif. Happens for all gifs and is visible after capturing the screen. See attached.

No green bar, because there is none on my screen.
None :(
After a restart, I was able to have 1 recording without the green bar. A little later, it is back.
This sounds like https://github.com/sindresorhus/Gifski/issues/64
I'm closing this issue as it's upstream and will not be fixed by us, as mentioned in the issue @sindresorhus linked you should be able to work around this by making sure the recording dimensions are divisible by 4.
sindresorhus/Gifski#64
Thanks a lot, this was indeed the issue! Have resolved it using the suggested solution.
Most helpful comment
This sounds like https://github.com/sindresorhus/Gifski/issues/64