Hi there,
you would need to find out which Litespeed setting is breaking it.
It could be either the way it is combining CSS or more likely if its combing Javascript.
If its a JS issue then check with litespeed on how to exclude scripts from whatever optimizations its doing.
These are locations of the 2 menu scripts that GP uses, try excluding those.
/wp-content/themes/generatepress/assets/js/menu.min.js
/wp-content/plugins/gp-premium/menu-plus/functions/js/offside.min.js
But you may want to also ask Litespeed support on this