Just as you thought. It is a message from Vriska summoning you to do battle. It is written in Gamzee's rich jelly-smelling blood, but it is her handwriting for sure. Her quirk is present, leaving the matter unde88a8le.
But why the hell does she want you to bring this stupid puppet along? She's so weird.
There is a journal entry on the back. It probably isn't the slightest bit relevant to the current situation, but you guess it couldn't hurt to read it anyway.