In the rich text editor:
@Gull
In a comment:
[Gull](https://www.worldofgaia.org/user/Gull)
For Names and Avatars:
In the rich text editor:
%Gull
In a comment:
[](https://www.worldofgaia.org/user/Gull) [Gull](https://www.worldofgaia.org/user/Gull)