Broadway

Being around theatrical people, Gabe's family had more casting news laid on them than they wanted to hear, and Gabe became a performer through happenstance. "Somebody said to my mother, 'They're lookin' for a kid for the play 'Dead End', why don't you take Gabe over?" and my mother said, "Why not?" says Gabe. Taken from an interview he did in a 1972 TVGuide. Before he got into "Dead-End," he played on Broadway with famous Alla Nazimova, but it was "Dead End" that turned him and the other boys into celebrities.
Here are the names of his On and Off Broadway Plays that I have gathered thoughout the internet. They are not in order of the appearance that he starred in, they are:

Dead End

Bye-Bye Birdie

The Prisoner of Second Avenue

Adaptation

Next

One Flew Over the Cuckoo's Nest

Anyone Can Whistle

Lamppost Reunion-- Which he was nominated for a Tony Award

Luv

The Sign in Sidney Brunstein's Window(This Was His
Favorite Role)

Fortuna

The Automobile Graveyard

Man Out Loud-Girl Quiet

Oklahoma

Can-Can

Wonderful Town

Ankles Aweigh

Little Me

Marathon '33

Tickets Please

Armada

Chocolates

Fun City

Where Do We Go From Here.

Gabriel Dell in latter years went behind the scenes instead of in front at some small theaters and playhouses, directing talented young actors with the expertise that only Gabe could teach..