Actually, and I could be wrong on this but I don't think so, he can sign anywhere via a NHL contract.
The NHL contract would trump the AHL one, and once he signs a NHL deal he can be sent to the minor league affiliate of the NHL team, not Manchester.
For example, if Ottawa signed Gravel to an NHL deal but felt he needed time in the minors they would assign him to Binghampton, not to Manchester.
Also there's likely an escape clause to the AHL deal with Manchetser that it becomes null and void in the event Gravel signs an NHL deal, similar to what you see in players who sign on the play in Europe. I also beleive that Mathias Niederberger, who was signed to an AHL deal in the Kings organization, has the same out clause.