Is the Grand Theft Auto V singleplayer DLC out yet?
NOPE
Rockstar wrote in a blog post from December 2013:

"For those ready to jump back into the story of Grand Theft Auto V, we have big plans for substantial additions in 2014 continuing Michael, Franklin and Trevor's action, mayhem and unexpected adventures in Southern San Andreas."


...and no sign of singleplayer DLC yet.
Get your shit together R*.

Sincerely,
The entire GTA community