I was reading Recipe for Gertrude, and in chapter 10, there's a part where they're driving in a car and one of the characters says, "Now sit down please. That could be fatal in this manga." I was wondering if this was a relatively common occurrence. Also, I was wondering where that idea of mentioning in a manga that they're in a manga came from.
This is indeed a common trope, and not even a Manga specific one, generally referred to as Breaking the Fourth Wall.
For example the Marvel character Deadpool is a well-known for doing this in an even more extreme fashion. You can find a lot of Manga/Anime examples here.
To learn more about the different versions of Fourth Wall Breakage visit this article on the Sliding Scale of Fourth Wall Hardness.
To my knowledge, in this case it would count as Fourth Wall Observer.