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”