The answer depends on wich platforms you're developing the phongap app, and if you're following the standard directory structure.
If your project directory structure is standard, then you can start from this gitignore and modify it for your needs.
On a rule of thumb you've to exclude all generated files like the bin/
and gen/
directories.
If you're developing an Android version of your app you should exclude build files too like *.apk
.
All generated files in the android
subdirectory should be excluded too:
Android/bin/
Android/gen/
Android/assets/