Google Drive replaces the leading dot in dot-filenames with an underscore. To undo this after downloading the files as a zip run
  # replaces leading underscores with dots
  rename -n 's/^_/./g' *  # first check
  rename 's/^_/./g' *  # now execute
After downloading the files be sure to set the correct permissions
chmod 700 ~/.ssh
chmod 644 ~/.ssh/*.pub
# update private files to 600