Erich Gamma worked for Object Technology when VisualAge was being developed so using XP/Agile before the XP book does not surprise me.

I worked with Erich before Visual Age when he developed the Presentation Framework. He came up with a process identical to XP. When XP came out I realized that Erich and Kent Beck were friends.

Erich is so prolific that he doesn't remember everything he invented. He is also generous.