Brad Dutton has been leading a community that is continuing Gallery development. He and his team have made progress over the past year and have a stable version of Gallery 3 that is PHP 7+ compatible, contains a replacement for the old Flash-based image uploader, an updated jQuery Library and other features. They’ve been testing it for a year now with no problems and have released it as Gallery 3.1.0.
This needs to be installed for WordPress and is not necessarily installed by default. Gallery might need it as well but since no images have been added since brandylion.com went into its own vm, I haven’t noticed any problems.