David Franzoni (who first pitched the idea) wrote the first draft of Gladiator
. Ridley Scott then hired Logan to rewrite it. Most of Logan's revisions dealt with the dialogue and the first act, and it was his idea to kill off Maximus's family. The script was again rewritten, this time by William Nicholson, and then David Franzoni came back and revised a lot of Logan's and Nicholson's rewrites. Then the script went through various rewrites/revisions during filming.
So while Logan did contribute to the screenplay, he wasn't the only writer, nor was he even the one who came up with the idea, even though he seems to have gotten the majority of the credit.
However, Logan has also contributed to the screenplays for The Aviator, The Last Samurai,
and Sweeney Todd
, so I'm looking forward to seeing what he brought to Skyfall