By default, GitKraken is installed in the local application data folder. You can find the executable inside %LOCALAPPDATA%\gitkraken\app-<app-version>
Starting with this entry, the latest version is 2.0.1, so exe will be %LOCALAPPDATA%\gitkraken\app-2.0.1\gitkraken.exe
And an easier way to open the software is to use the shortcut in the Start menu:

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