moving mail folders to gmail labels - sublabels and hierarchy is lost

I use Mail.app and iOS Mail to access 3 different gmail accounts. Currently I have all of my mail in folders "on my mac", but I'd like to be able to access them from my mobile devices (iphone, ipad) or when logging onto email from another computer (using the webapp). When I drag and drop folders into my gmail accounts (as per this answer), the nesting/sublabels/heirarchy is lost. How can I migrate preserving the proper structure?

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My Folders looks something like:

Group/
|- SubGroup1/
|- SubGroup2/
   |- SubSubGroup/
   |- <some messages>
|- <some messages>

and the new gmail labels look like:

Group (messages)
Group/SubGroup1
Group/SubGroup2
Group/SubGroup2/SubSubGroup
Group/SubGroup2/SubSubGroup (messages)