How do I fix "The plugin "External Tools" could not be loaded. An error occured: Plugin loader "python 3" was not found)" error in GEdit?

You must and should rollback to default Python version, i.e., 3.8.2 in 20.04 since there are some libraries which are required by some packages installed in the system only for the default version of Python that may not be available or compatible with other versions. Since you've installed Python 3.8.3 from source, the files were copied in /usr/local/ and /usr/local/bin/python3 is created which a symlink, a.k.a, symbolic link or soft link, of /usr/local/bin/python3.8.

The default value of PATH contains /usr/local/bin before /usr/bin, thus given priority and the reason behind invoking Python 3.8.3 instead of 3.8.2 whenever python3 or python3.8 is called (since default Python is installed in /usr/bin).

To revert back to the default python3 won't take much time for you. You have 3 options to revert back.

1. Removing the symlink: Just delete /usr/local/bin/python3. Yes that's it.

2. Recreate symlink: You can make /usr/local/bin/python3 to point to /usr/bin/python3.8 or /usr/bin/python3 (which is already a symlink to /usr/bin/python3.8 as per your message in general room).

   To recreate symlink, use ln with s and f options, i.e.,

   sudo ln -sf /usr/bin/python3.8 /usr/local/bin/python3 # You may replace /usr/bin/python3.8 with /usr/bin/python3 as explained above
   

3. Giving /usr/bin priority (overkill): Edit /etc/environment and place /usr/bin before /usr/local/bin.

   Warning!!! You should handle /etc/environment with care. Don't touch or do anything unless you know what you're doing.

   Alternatively, you can place PATH="/usr/bin:$PATH" in /.profile.

   Once done, logout and relogin to make changes take place.

Since you've installed Python 3.8.3 from source, you need to delete files manually as Python don't provide uninstall option for make. To delete them run:

# From /usr/local/bin
cd /usr/local/bin
sudo rm 2to3 2to3-3.8 easy_install-3.8 idle3 idle3.8 pip3 pip3.8 pydoc3 pydoc3.8 python3 python3.8 python3.8-config python3-config
# From /usr/local/include
sudo rm -r /usr/local/include/python3.8
# From /usr/local/lib
cd /usr/local/lib
sudo rm -r libpython3.8.a pkgconfig python3.8
# From /usr/local/man/man1
cd /usr/local/man/man1
sudo rm -r python3.1 python3.8.1
# From /usr/local/share/man/man1
cd /usr/local/share/man/man1
sudo rm -r python3.1 python3.8.1