AFIK, the maximum gas price is 500 Gwei as defined in https://github.com/ethereum/go-ethereum/blob/master/eth/gasprice/gasprice.go#L32

var maxPrice = big.NewInt(500 * params.GWei)

Not sure about the overall maximum gas price but when you use go-ethereum client and ask for a SuggestPrice, there is a cap set to 500 Gwei as defined in https://github.com/ethereum/go-ethereum/blob/master/eth/gasprice/gasprice.go#L32

var maxPrice = big.NewInt(500 * params.GWei)