I read a post in this forum about symmetry but it is kinda laggy.
Here is how we can do it with better speed :
- Put guide in the middle of the document with ALT,
- select left area with selection tool and Alt to follow guide,
- use bucket to put color into the selection,
- dupplicate this layer to a virtual layer,
- make virtual layer be symetric, layer > rotate layer > horizontal symmetry,
- select the original layer with the black arrow,
- unselect all (Ctrl-D),
- select brush tool,
- paint inside,
- enjoy
Excuse my french ^^
EDIT : modified subject, wasn't clear.