77cm puts them at 40kgs - which is absolutely fine for GG's. I would continue with your current regime, but slow it down a bit in autumn.
Check the covering on their back, see if you can feel the vertebrae. You don't want a rounded back. GG's put on fat very easily, but goatlings need a good diet to continue to grow. Also if you look from behind down the spine, you don't want to see that their shoulders are as wide as the hips, but often they are at this age. If yes, I would take out some of the protein from their diet.
I keep mine on full rations until August, and then they go onto mainly hay with a handful of soaked shreds and a dusting of oats until 8 weeks before kidding. They still get branches (as long as available) and carrots over winter.
But I still wouldn't mate them to anything bigger than a GG for their first time.