How much fast food would have been available in Gidget's time?

In this episode, Gidge takes off with the creep and when they park, he has an ice chest loaded with sandwiches and other foods.

Doubtful today's Cassanovas would do that, but rather take their "date" for fast food on the way to their parking area.

Fast food was certainly around, though it was mostly local chains.

Yes, there were plenty of fast food hamburger places back in the good old days (the 1960's, yipe, seems like a couple years ago). For one I remember the original McDonalds with the big golden arches all white building and Ronald McDonald with the McDonalds tray on his head for a hat. And right down the street from my grandmother's place was the Burger King that showed a King sitting on top of a burger with a milkshake. Little Tavern with the "bag of burgers" was also around (similar to White Castle type burgers).
I'll try to include the photos here, if it doesn't work you find Burger King here:
https://i.pinimg.com/originals/48/a8/21/48a821cc4161ed4c53d962e40189c9a4.jpg

https://www.sitcomsonline.com/boards/attachment.php?attachmentid=307270&stc=1&d=1718758750

Another franchise that started in the 1950s and grew rapidly: Burger Chef, a significant player during Gidget's time.

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It's claimed they were in 39 states by 1968, so of the lower 48 only 9 were without at least one. Their growth was actually bigger than Burger King by the early '70s, behind only McDonald's.