Was Solomon alone when he sacrificed 1000 burnt offerings at Gibeon?

---

Yes

(At least no one else is mentioned)

And the king went to Gibeon to sacrifice there; for that was the great high place: a thousand burnt offerings did Solomon offer upon that altar. 1 Kings 3:4

No, everyone in Israel was with him.

Solomon, and all the congregation with him, went to the high place that was at Gibeon. ...

And Solomon ... offered a thousand burnt offerings upon it. 2 Chronicles 1:3-6