I've suffered with this problem too. But for me, sometimes it works and sometimes it doesn't.
When a disc is burned and the 2 second gap is there, it was burned Track-At-Once (TAO). When the 2 second gap is not there, the disc is burned Disc-At-Once (DAO).
I think you can also have no DAO if your cdr burner is not supported by MJ. I've asked here before for a valid list of supported burners, but no one seems to have one. MJ uses the CDRWin burning libraries, but which burners are supported depends on what version of the CDRWin libraries the current build of MJ was linked with. So at this point, who knows what burners are supported.
But anyway, sometimes I can uncheck the 2 second gap and other times it's greyed out.
I also have found that a reboot works to restore my ability to uncheck the 2 second gap check box.
I think this is related to whether or not I've run Nero or not. I think for me, it's some dll conflict between Nero and MJ, but who knows? At least a reboot seems to fix it, so I can live with it.