“Grandma pizza”/“grandma slice” was popularized on Long Island and is a square pizza with cheese and tomatoes. There usually is not “grandpa pizza,” but this has been suggested as a counterpart to “grandma.”
   
“Grandpa pizza, topped with ground sausage and broccoli rabe,” served at Gino’s on Kissena, was printed in Newsday (Long Island, NY) in 2001. “Prince Umberto’s, they make a Grandpa pizza, very thin-crusted with fresh tomatoes and fresh mozzarella” was printed in Newsday (Long Island, NY) in 2006.
 
     
Wikipedia: Pizza in the United States
Grandma pizza is a thin, square pizza, typically with cheese and tomatoes. It is reminiscent of pizzas cooked at home by Italian housewives without a pizza oven, and was popularized on Long Island.
           
8 June 2001, Newsday (Long Island, NY), “Food Day” by Kim Rose, pg, B23:
GINO’S OF KISSENA, 65-01 Kissena Blvd., Flushing, 718-939-4474, is the third in a chain of Italian pizzerias that started on Long Island- in Great Neck in 1980 and in Manhasset in 1985. (...) Special house pies include the Grandpa pizza, topped with ground sausage and broccoli rabe, and a low-cal, no-cheese salad pizza, with fresh lettuce, tomatoes and slices of grilled chicken.
 
Google Groups: rec.food.cooking
Great Pizza Grandma’s & Grandpas
CLIFF
5/16/02
I live in Corel Springs Florida. A new kind of food/restaurant/ice cream store just opened called “Slice n’ Ice”. It’s a pizza place / ice cream store. Never saw one like this before.
It’s the best pizza I’ve had in years. They have something called Grandma’s & Grandpa’s pizza - a rectangled shaped pie. Grandma had marainara and fresh mozzarella - Grandpa had just marinara & parmeasan cheese.
         
5 March 2006, Newsday (Long Island, NY), “Who’s Cooking” by Joan Reminick, pg. G17:
There’s a pizzeria not too far from my house, Prince Umberto’s, they make a Grandpa pizza, very thin-crusted with fresh tomatoes and fresh mozzarella. It’s got a very good taste cold.
       
To Live and Eat in NYC
WEDNESDAY, AUGUST 6, 2008
The Grandma and Grandpa Pizza Pie
Driggs Pizza & Italian Restaurant’s motto is “Serving Italian Home Cooked Meals Since 1968.
(...)
The Grandpa slice is almost as good. It comes with a ton of garlic, diced tomato, basil, cheese, olive oil, and a bit of spice. Also on a toasty crust.
 
7 November 2008, Newsday (Long Island, NY), “Weekend Eats” by Peter M. Gianotti, pg, B31:
Emilio’s
2201 Jericho Tpke.
Commack
Emilio’s is a triple-feature starring pizzas, pastas and hearty Italian main courses. Just as tasty are the salads, panini, wraps and frittatas. It’s all G-rated, too. Some favorites: pasta e fagioli, Old World marinara pie, the grandpa pizza and baked pastas.
 
Twitter
Tyler Cordaro‏
@tylerc66
@summersm you have to get the grandpa slice. Its like a grandma but instead of red sauce they use vodka sauce.
10:18 PM - 25 Feb 2009
     
Twitter
Arthur Bovino‏
@ArthurBovino
GIGANTE’S OF OCEANSIDE: Grandma Pizza, Country-style Grandpa Slice, Grandpa Slice w/Vodka Sauce.
7:51 PM - 19 Jun 2010
   
Twitter
FredDexMS (Juiceman)‏
@FredDexMS
Follow Follow @FredDexMS
@BullfrogAndBaum Prince Umberto’s Grandpa slice in Franklin Square LI! http://myloc.me/bfIfs
2:03 PM - 30 Aug 2010
   
Twitter
Larry gEE‏
@_larrygee_
Right around from where I’m staying in Brooklyn is my favorite Pizza spot “Driggs Pizza”.  The “Grandpa Slice” is… http://fb.me/1l7JlW5xG
9:31 AM - 21 Oct 2011
   
Twitter
FredDexMS (Juiceman)‏
@FredDexMS
@LosVascos Le Dix w/ Long Island Grandpa slice!!! #hibrow #lowbrow pairing!!! http://instagram.com/p/ZL1OBVoECP/
3:30 PM - 11 May 2013
 
Twitter
Ahoy NY Food Tours‏
@AhoyNewYork
A grandpa slice from Grand Appetito. Fresh tomatoes, fresh mozzarella and fresh basil!
9:45 AM - 2 Jul 2013
 
Twitter
Laura Chant‏
@lkchant119
I’ve always known about grandma pizza but since when has grandpa pizza been a thing?
8:05 PM - 16 May 2014
         
Twitter
Samantha Casper‏
@S_CASPER4
A grandpa slice ; breadcrumbs cheese onions 😍 legit looks like heaven
12:41 PM - 18 Mar 2015
 
Twitter
Matthew Borgatti‏
@GiantEye
The grandpa slice includes toasted pepperoni and garlic. I approve. @ Vinnie’s Pizza https://instagram.com/p/0qjsPOvmr-/
3:59 PM - 25 Mar 2015
 
Twitter
Chris Cullinan‏
@therealculley
@TestKitchen @cpkimball When you make Grandma Pizza on the grill, does it become Grandpa Pizza?
6:06 PM - 21 Aug 2015
 
Twitter
Daniel‏
@aeche
The grandpa slice: a grandma slice with sausage and cigarette ash.
5:51 PM - 16 Sep 2015
 
Reddit—Shower Thoughts
Grandpa pizza should really just be a Grandma pizza with meat on it.
submitted October 14, 2017 by the_bison