Gwendolyn "Gwen" Poole is a woman from the planet Earth where characters like Wolverine and Magneto appear in comics published by Marvel Entertainment. Through an event she prefers not to talk about, she was transported into the comic books.
After arriving, Gwen did not want to be an "extra" and applied for a costume from a tailor that specializes in supporting super-heroes. The tailor misread her name as the alias "Gwenpool" and created a costume similar to Deadpool's, using Gwen's pink hair highlights as the starting point, her request to have the costume stand out, and to use up pink fabric on hand.
Gwenpool used her knowledge of comic book characters to her advantage, knowing her actions don't have any consequences, and helping to make up for the fact that she does not have any training or superpowers. During her adventures, she gains the power to move into the whitespace between the panels in comic books, and later interacts with characters on previous pages and issues.
After the character Ghost-Spider (later called Spider-Gwen) became popular with readers, Marvel released variant covers of their titles featuring Gwen Stacy reimagined as other characters like Doctor Strange and Groot. The alternate cover of Deadpool's Secret Secret Wars #2 featured Gwen Stacy in a white-and-pink Deadpool-style costume, relaxing in an inflatable chair floating in a swimming pool.
Marvel editor Jordan White gave Christopher Hastings and Heather Antos the task of creating a story based on the cover artwork. Like Deadpool, Gwenpool has the ability to break the fourth wall. However, this ability comes from being transported from her previous world into comics, rather than from the mental condition that allows Deadpool to know he is in a comic book.