that Gandalf was saying that the benefits to Gwaihir from the third journey would be repayment for all the journeys. That is, that Gandalf is paying Gwaihir for the rides with the outcome of the third trip.

--------
The Hero

‘Jack fell as he’d have wished,’ the Mother said,  
And folded up the letter that she’d read.  
‘The Colonel writes so nicely.’ Something broke  
In the tired voice that quavered to a choke.  
She half looked up. ‘We mothers are so proud          
Of our dead soldiers.’ Then her face was bowed.  
  
Quietly the Brother Officer went out.  
He’d told the poor old dear some gallant lies  
That she would nourish all her days, no doubt.  
For while he coughed and mumbled, her weak eyes   
Had shone with gentle triumph, brimmed with joy,  
Because he’d been so brave, her glorious boy.  
  
He thought how ‘Jack’, cold-footed, useless swine,  
Had panicked down the trench that night the mine  
Went up at Wicked Corner; how he’d tried  
To get sent home, and how, at last, he died,  
Blown to small bits. And no one seemed to care  
Except that lonely woman with white hair.

-Siegfried Sassoon
From "The Old Huntsman and Other Poems", 1918