Before Copy Runtime Geodatabase to File Geodatabase tool existed in ArcMap (10.2 and earlier), we had to use a combination of two tools instead. These steps should also work in ArcGIS Pro:
1. Search for the tool “Export XML Workspace Document”
2. In Input data, drag and drop the .geodatabase file from your device (you have to drag and drop it in the white line below “Input data” because you won’t be able to find it using the “Open” button)
3. Choose a location for the output .xml file. Leave all other options by default.
4. Once done, open the tool Import XML Workspace Document (in Data Management/Distributed Geodatabase)
5. In Target GDB, put the location of the geodatabase that will contain all the exported data (it will automatically create one if it doesn’t exists yet in the specified folder). Always use a blank GDB.
6. In Import file, put the .xml file you created earlier. Leave other options by default.
7. You should now have a file geodatabase containing all the data
Here's where I found the original steps that I modified for Pro: Tuto: Collector for ArcGIS: Go (really) offline! | CartoBLOG
I tested this out in Pro 2.0 and it looks like this works. Hope this helps!