A boy gets a gun and a disc with a star on it from an old man. He shoots it into the ground, the disc makes a circle, and a robot emerges from the ground. The boy is inside a robot and controls it by moving his body. with him. There is also a baby girl with bunny ears. It’s a Japanese mecha anime from the 80's.
This is Madö King Granzört, which aired from 1989 to 1990.
In 2050, the moon was mysteriously terraformed after a great earthquake. Fifty years later, the moon is now populated with humans and has become a popular tourist destination. After a boy named Daichi arrives on the moon for summer vacation, he accidentally meets an old witch, V-Mei, and her granddaughter Guri Guri. Both of them are members of the endangered Long-Ears race, currently at war with the evil Jadou Clan, who are also planning to take over Earth. V-Mei gives a magic gun to Daichi and reveals that he is the chosen Madou Warrior destined to save the Long-Ears race and their land Rabiluna from the Jadou Clan.
With the magic gun, Daichi can summon Granzort, a giant robot called a Madou King with the elemental power of Earth, to fight against the monsters of the Jadou Clan. During their travels, Daichi, V-Mei, and Guru Guri are later joined by two other Madou Warriors, Gus and Rabi. Gus's magic bow summons Winzart, the Madou King of Wind, while Rabi's magic top summons Aquabeat, the Madou King of Water.
I found it by searching for mecha anime gun disc star summon, which brought up Mecha anime with giant robots summoned from sky, water and earth