The rules for the Manx are very clear and are in the regs every year.

And I quote..........

"The only riders eligible for the Newcomers Race and the only riders regarded as newcomers in the Classic Races shall be those who have not previously qualified to ride or ridden in any motorcycle race run over the TT Mountain Course. A rider who has previously competed on the TT Course only as a TT sidecar driver or passenger is regarded as a Newcomer in the Manx Grand Prix. Riders entering the Newcomers Race may also enter for the Junior, Lightweight, Ultra Lightweight or Senior races. No rider may compete in the Junior, Lightweight, Ultra Lightweight or Senior Races unless he/she has previously qualified to start in a race over the TT Mountain Course.
In these regulations and in any other instructions or directive issued by the Club, the term "rider" shall refer to any person who has been accepted as a rider in any of the races.
Riders must be over the age of 18 years on 18 August.
For the purposes of these regulations, a rider is a person who has not ridden in the Tourist Trophy Races, except as a sidecar driver or passenger, or any round of any road racing world championship, including practice. This restriction shall not apply to riders in the Classic Races and shall not exclude from any other race riders who have competed only in the Production or Historic Motor Cycle Races on the TT Course. Riders in TT Production races, who have also qualified for and competed in the Senior TT on a production machine, may also compete in the Manx Grand Prix provided they have not competed in any other TT race. Riders who have practised for or raced in TT Superstock Races are not eligible for the MGP (except Classic races)."

The gentlemans agreement applies to Manx GP winners who normally move on to the TT. I have not yet heard of a Manx GP winner trying to break that gentlemans agreement by submiting an entry for the Manx.
I suspect he/she would be refused.