You will need to re-generate your config.yml or add this to the bottom.
# Other Player Message, the message that comes up when you change other player's gamemode.
# You can use color Codes.
# Use %gamemode% to get the player's gamemode that has been changed.
# Use %player% to say the player's name that the gamemode that has been changed is.
otherplayer: "&a&l(!) &aYou have changed &9%player% &ato the gamemode &b%gamemode%"