How to delete multiple branches in GitKraken?

You can delete multiple branches when they are inside a folder.For example feature/add-search

That way, you can right-click the feature fold and delete all branches which are inside the folder.

I do not think there is a way to delete multiple branches at once in GK at this time.

You might want to use a terminal like git bash for this (File -> Open Terminal in GK if a terminal was set up in Preferences). See this answer on how to delete multiple branches with one command via command line.