So up till he left for training the only way he could summon a high level toad like Gamma was if he was in a life for death situation or someone he cared about was in danger. Up till then he ended up with a tadpoll or a small toad every time he tried.
But Training with Juraiya all that time he had to have worked on his summoning technique more. But I'm just wondering how much he improved his summoning. Who knows. Maybe he'll summon something during the fight with Tobi.