It's really hard to say who the gunmen actually were. Obviously with something this ambitious the men Capone would have chosen would be some of his most loyal soldiers.

Former FBI Agent Bill Roemer Jr. believes that the four shooters were "Machine Gun" Jack McGurn, "Little New York" Louie Campagna who were dressed in the police officers uniforms and Tony "Joe Batters" Accardo and "Tough" Tony Capezio who carried in the Thompsons. Although he's never provided much proof to this statement, he's held firm to it in a couple of his books.

Despite his "Blond Alibi", one thing that may be taken into consideration as evidence of Jack McGurn being there is a clue left at his own murder on February 15, 1936 , A Valentines Day card that read:
G
G
You've lost your job,
You've lost your dough
Your jewels and handsome houses
At least you have your trousers.
G
G