As I mention in the post, there is a limit to the amount of gas a block can have, so the maximum gas a transaction could have and still be accepted is limited by this block size. See https://ethstats.net/ "gas limit" section to see the block gas limit (it's dynamic).

There is no maximum gas price, but it's bounded by the largest number you can put into that field. IIRC, every number is 128 bits, so it would be 2^128 - 1.