It's not what the buttons look like, it's what they do. In Krita, making an ellipse involves clicking the ellipse button and dragging it somewhere. You now have an ellipse, and you hold shift if you want to make it a circle instead.
In GIMP there is no direct ellipse tool, there's only an ellipse select tool, likewise you hold shift to make it a circle. Then you use a menu item to select the border of your selection, getting a popup to let you determine how much pixels you want. And then, you use the fill tool or fill menu item to fill it. That's a surprising amount of clicks to accomplish what's most likely the single most common task for anyone opening a screenshot in an image editor. I'm not aware of any easier/faster method to do it. Feels like it should exist, but this is also what you get if you search for how to draw a circle in GIMP, so if it exists everyone's missing it.
GIMP's method gives you more power, but you rarely ever need that power. But when you do, Krita also has ellipse select, border select and various fill tools that can be strung together in the same way.
Given that the UUID changed, you almost certainly made a new LUKS container, overwriting the old one. That's bad, because the LUKS header is the only source of the actual encryption key that was used, and making a new one will overwrite both the main header as well as its backup copy immediately. Your password/keyfile/whatever is merely used to decrypt the part of the header that has the actual encryption key, and that's gone in that case.
Unless you have access to a header backup from before that, there's a fairly strong chance it's irrecoverable. I'd suggest going through any archives you might have to see if you have such a backup - most of the instructions on the Gentoo wiki encourage making one, so you might have made one through the power of copying & pasting instructions. Should be a file of around 16MB.