When the barons were summoned to Oxford in June 1233 and refused to meet with the king's Poitevin relatives, he took a very prominent part in their councils; so much so that, according to Matthew Paris, Henry's wrath was specially kindled against him.
At the same time Richard Siward, Gilbert's nephew by marriage, was seized by the king's orders and held captive—presumably as a hostage for his uncle's conduct.
When, on the advice of Stephen Segrave, Henry summoned Gilbert Basset and the confederated nobles to meet him at Gloucester in August 1233 and they refused to come, they were promptly outlawed, and orders were given for the destruction of the towns, castles, and parks belonging to them.
In retaliation for this, Basset and Siward set fire to Stephen Segrave's villa of Alconbury, though the king himself was then staying at Huntingdon, some four miles distant.
While he was going out to hunt, his horse tripped on a root and threw its rider, who was taken up in a kind of paralysis ("dissipatis ossibus et nervis dissolutis"), from which he never recovered.