男はグッと堪えて背中で語り
グッと堪え >堪える(こらえる koraeru)=endure bear stand
背中で語り >背中(せなか senaka=back)で 語る(かたるkataru=talk)
It means " Man should talk by his back with standing,endureing,bearing everything."
But acutually he can't talk whith his back.
So it means man shouldn't talk a lot (or anything). Others are watching man' s behavior,action and those are more than talking.
Here is the CM talk abou "背中で語る”
https://www.youtube.com/watch?v=1a_Wr3NLQn4
enjoy!