The big Spider-Man story of summer 2015 was "Spider-Verse." Leading up to this story, a series called Edge of the Spider-Verse spent each issue introducing a Spider-Man from a different alternate universe. Edge of the Spider-Verse #2 introduced a world where Gwen Stacy, not Peter Parker, was bit by a radioactive spider. This issue was wildly popular as soon as it was announced, sold out 5 printings, and she got her own series.
Her first appearance in Edge of the Spider-Verse #2 is the beginning in the first paperback below, so you can read Spider-Verse after that, or pretty easily ignore it completely. It's not really Gwen's story. You just need to know she lives in a different universe from the normal Spider-Man, and once she got recruited to help a ton of them fight some big bad guys.
Below of every Spider-Gwen paperback in reading order: