can i use caston grey on reforcment event character on game?

Not anymore.

As written, the only target requirement is that the character chosen be controlled by an opponent. Reinforcements, when in play as characters, are considered to be characters - and only characters - so they would qualify.

However, with the new errata (“...to choose an opponent’s character of equal or lower printed cost ...”), you cannot. As characters, the Reinforcement cards have no printed cost, so a comparison to the printed cost of the Martell NOBLE cannot be made, making the Reinforcements ineligible to be chosen as targets.