Heisei Gamera against Toho monsters

gameras mangna blast is powerful but theres no way he could beat them all in one shot

1 on 1 gammera could take out certain monsters like anguirus or baragon but stronger monsters like Godzilla, Ghidorah, or Mothra can defeat him

As sad as it makes me because I love Gamera, no. There are many he could battle 1 on 1 and come out on top easily..But when you're talking about characters such as Godzilla, Monster X/Keizer Ghidorah, King Ghidorah, Biollante, Mothra, and perhaps Destoroyha...I think he would get killed by them. 

Mothra? Gamera would destroy Mothra

It depends. He could win quite a bit of 1 on 1 battles, but some 1v1s would be lost by Gamera. 

I would put Gamera right up there with the top monsters. In a 1 on 1, I think he could end up winning against any Toho monster. He would't have the odds on his side against Ghidorah, MechaGodzilla, Space Godzilla, or Destroyah, but I don't think you could just write him off entirely. Gamera's kind of a bad*** when he needs to be.