Gilbert died in 1295 and therefore could not have served Edward II.
Bannockburn was in 1314. He was the Earl of Gloucester his father was
the Richard Earl of Hertford. Sorry, de Clare was not a St Clair. Also,
his name is not mentioned in my Mackie's History of Scotland. Please
note, all this is in Wales. Clare stems from the castle in Suffolk,
seat of the family.
quoted from http://www.castlewales.com/clare.html
"Earl Gilbert died at the age of 52 in December 1295, and his estates
were administered by his widow until her death in 1307. The young heir,
another Gilbert, was killed at the battle of Bannockburn in 1314. His
untimely death meant the extinction of the de Clare male line, and the
Gloucester inheritance was divided among three sisters.