In the email group discussing Design Patterns before it was published, I remember being amazed to find people who really had a sense of design. So much design advice I'd seen before that boiled down to a "miracle in the middle," but here were people exploring forces and solutions. "Composite" was the one that really brought it home for me: it was a technique I used sometimes, but the Design Patterns authors called it out with a real name and examined the forces around it.
I can't remember if it ever came up on the mailing list, but I've wondered since then if "Null Object" was a conscious omission, or left out due to a lack of consensus. (Bobby Woolf probably couldn't believe his luck; he wrote it up for PLoP-96.)
I don't know Erich Gamma or Richard Helm, but Ralph Johnson and the late John Vlissides have always been gracious people more interested in building a community than in basking in the well-deserved fame from their work.
You sometimes meet people who only know one pattern from the book—Singleton—but the truth is that this book has really had broad impact in the software community. It popularized important ideas, it spawned much other pattern work, and it has influenced many developers to create better designs. Congratulations to the authors on this anniversary!