It seems to be impossible to checkout commits with GitKraken.
As you can see there is no option to checkout older commits, only Cherrypick commit
but I don't know what this is good for.
EDIT: I was able to checkout with another git program (git bash) via console:
Now it looks like this in GitKraken:
They finally implemented this feature yesterday (Sept. 25th, 2019).
You can make a UNDO in the top bar. Anothe option can be reset to another commit using the right click on the commit.
GitKraken (as of version 1.7) does not seem to offer a way to get into a detached HEAD mode, and check out a commit directly. Similarly, it will also not allow you to check out a tag (since that would also give you a detached HEAD).
So instead, you will have to create a (temporary) branch first which you can then check out.
The way you do this is by right clicking on your desired commit and select "Reset to this commit" and then choose "Hard - Discard all changes"