That happens sometimes. The author has to work from an early version of the script, and if a scene gets cut at the last second, it's possible for it to still be in there. In this case, though, I think the entire script was rewritten and thrown out. But they didn't bother having the book rewritten, possibly because, seriously, who cares what's in the novelization of Gidget Goes to Rome?
The differences start in the very first scene. In the book, it's described this way: "And it was there I was lying one July noon last year, pretty much as God made me, except for a few minor accessories like a black bikini and a pair of sunglasses." In the movie, it's a red bikini. And no sunglasses. Okay, that's pretty small. But as soon as she gets to Rome, the book describes an entirely different series of events. Sure, the overall outline is the same (she fights with Moondoggie and has a crush on an older Italian man), but every element is different.
So with that out of the way, I want to go back to that part about "pretty much as God made me". Is that a reference to nudity? In a Gidget book? Oh my yes. Here's the scene after Gidget has dived into a fountain (which happens at the beginning of the trip in the book, as opposed at the end of the book for a totally different reason in the movie) and is getting examined by an older doctor, who will later turn out to be married, with children, and stringing her along:
With an air of detached sovereignty he inspected the thermometer, smiled knowingly and then asked me to take off my p.j.'s. First I got purple as a plum, but then it came to me that I had nothing to hide. While not as generously endowed as Lucy and Libby, I am endowed. Besides, he was a doctor, wasn't he?
Off came the top of my p.j.'s.
He placed his efficient hand on my back and began to knock around with the knuckles of his other hand in the general vicinity of my lower thorax. Then he bent down and affixed his ear to my back and asked me to breathe. Then he asked me to lie flat on my back.
I complied. My no-longer-hidden treasures pointed straight to the ceiling. Dottore Paladino again leaned his perfumed head over me and pressed his ear toward the region of my heart, just underneath my right half-bosom. I have been examined before, but never like this. It tickled.
I mean, really now! I can assure you that that did not take place in the movie I watched. Also, "right half-bosom"?
Another spot I found strange was when Gidget is pontificating on the difference between jet travel and boat travel. She's very worldly, this Gidget. She also talks in Latin a lot more than I would have expected. And here, what's with this section on her parents' drinking habits?
It's different with wine. We drink it at home, my folks making a sort of a ritual out of it, serving it at the right room temperature (red one), or slightly chilled (white one), and my ma being what is known as "a second plateau drinker" (steady intake of small sips from 6 p.m. onward), while my old man has reached "the first plateau" long ago (steady intake from 5 p.m. onward). But they never get bombed. Drinking wine just puts them in a benevolent mood.
I trust that it will not come as surprise that a Google search for "second plateau drinker" yields no results.