I am following instructions on wiring an LED screen to an Arduino. I have a Raspberry Pi 3 Model B, so I am having to "interpret" the instructions. I'm stuck on a step where the instructions say to connect to "Arduino Digital Pin 7". Is that GPIO 7 on the Raspberry? Is there a mapping somewhere of Arduino pins to GPIO pins?
GPIO pins are there for you to interact with other devices. The name/numbers they are given are arbitrary and depend on the system that you're using. As you can see by comparing the Arduino pinout with the Raspberry Pi pinout they have nothing to do with each other.
You should (in general) be able to use any digital GPIO pin to do what you want. It doesn't have to be pin 7. What really matters is that you connect your circuit to the pins that you specify in your code. In some cases, some GPIO pins serve another function, and you just need to make sure that they're configured to act as a GPIO pin.
I would suggest that before you go on with your project, you checkout this tutorial on turning on LEDs with a Raspberry Pi. Once the usage of GPIO pins on a Raspberry Pi is clear to you, it'll be much easier to build more complex problems.
As a side note, when you have questions regarding a tutorial, it's best if you provide a link to it, so that others may clearly understand what steps you're following.