You may change the font settings of a gitk window by making changes in Edit->Preferences. But these settings are reflected only for that session of gitk.
How can I persist the font settings for all further sessions?
You should see, when you quit gitk, a setting file created/updated in:
%HOMEPATH%\.config/git/gitk
It will include a line like:
set mainfont {{Calibri Light} 9}
Those settings will persists across sessions.