Gillingham chairman Paul Scally told the club's official website: "This is the best deal we can do at the moment and there is no guarantee we could do any better in the near future,"
King will discuss personal terms today after Marlon Harewood completed his £500,000 switch to West Ham United on Tuesday.
King already has five goals in 13 games this season and Gillingham's financial plight has forced them to sell their prized asset.
If King signs, the Gills still won't recieve all £950,000, his former club Barnet have a sell-on clause entitling them to around £190,000.