Those powers he gained only later on, if he had them, then he would not need to use GM and probably would save both Konan and Yahiko. The anger released his shinratensei, but only later on he gained others powers by studing his rinnegan. If it wasn't for Yahiko's death and summoning GM, he might not been able to ever use those powers. You mustn't forget, he wasn't like Sasuke, he didn't get the list of what and how his eyes can do other things.

It never really was mentioned when Nagato summoned the GM for the first time, as Konan knew what he was planing and tried to stop him.

It was probably similar with the summon path, he have seen the world wide and found countless summons and strenghtened them. They could be just like in Sasuke's case, just a normal animals, but thanks to rinnegan and the rods they were powered up.