It might help straightening out your problem if you can visualize just how your photos are stored on your phone.
The photo files that make up your photo library reside within a folder. At one time the DCIM folder (Digital Camera IMages) was the default name, an industry wide common standard so webcams, video recorders, mobile devices all used DCIM as the default save location.
But unfortunately proprietary interests messed that all up for us so now it's a matter of depending on the operating system version, the manufacturer, and, the app the default storage folder for photos might be named DCIM, or Photos, or Pictures, or Images, or whatever.
And you'll have multiple default save folders as different apps may be using different ones. That's the issue that you're encountering -- you're seeing your photos using the Gallery app since it's using its chosen default save folder(s) but the Facebook app is looking in a different folder as its default.
Once you take into consideration all that nonsense, that requires you to start doing a little sleuthing to determine just which folder your photos are stored in, and then you need to configure things so your Gallery app and your Facebook app are working off that same folder. I know you stated you're not too tech savvy but this will help you in the long run to be a lot more comfortable to just fix these kinds of quirks that will pop up occasionally. Get familiar with the various options in your Pixel's Settings menu, and with the Settings/Options/Preferences menu in the apps you use a lot. You can really optimize your phone to suit your particular needs. It may appear to be a daunting task, but just approach it one step at time.
Anyway, check if the Camera app has a default folder option in the app's Settings menu. That might point you to which folder. If it doesn't, try using a file manager app to peruse through the various folders in your phone, looking for DCIM or Pics or whatever that contains your photo collection. Once you find it, than you just need to get the Facebook app to detect and use that same folder.