Nope!
The editor will send you a list of things they want (Archetypes, Backgrounds, Heritages, Cultures, Feats, Spells, Alternate Class Abilities, Monsters, Etc) and you can pick and choose off that list anything you want to pitch. But that's based on the GPG Patreon poll, so you can keep track of where that is and plan pitches accordingly!
So far the only full class was in GPG 1 with the Artificer, but I'm hoping against hope we'll get another class entry that I can fight, tooth and nail, for!
But as far as the "Theme" it's entirely in the air. I've done two articles about an Airship crew, two Magic Martial Maneuvers articles, the Zevites as a planestriders heritage, a dungeon delving system, the backgrounds and destinies in the Edge of Night article...
And my next article involves Voidrunning space ant people.