You’re righter than you think. Historians have named a library’s worth of “first” science fiction works. Anybody who wants to make a name in the field can propose another and be equally certain that people will take a good case seriously. That’s because the genre of science fiction didn’t exist before the genre of science fiction was invented.
Am I being circular? No, not really. Verne and Wells and their contemporaries never thought of writing science fiction. They wrote mainstream fiction that explored the technological advances they saw around them every day. They cheated by extrapolating that technology a bit farther than contemporary reality but that wasn’t seen as a distinction from the exaggerations of other forms of adventure fiction.
Gernsback codified scientific romances and gave writers a structure into which they could place works that had few outlets elsewhere. That’s why he’s so critical to the history of the genre, even though the original works he published were vastly inferior to the mainstream stories that preceded them. Without his structure, the genre could not take form and improve.