I’ve recently been re-reading Morton’s Barber of Putney and Frankau’s Peter Jackson, Cigar Merchant. Both were published in 1919, and in both the Germans are sometimes referred to slangily as “Gerboys”.
I don’t think I’ve seen this term outside these two novels, and I wonder how common it was. I suppose it’s a way infantilizing the enemy, reducing “man” to “boy” and making him seem harmless – in a way that seems familiar, even affectionate. Rather like the use of “Fritz”, and very unlike the hostile “Hun”.