On 14th January 1933, Allsop played in the FA Cup tie against Arsenal, the First Division league champions. Injuries and illness robbed Arsenal of several key players including Eddie Hapgood, Joe Hulme, Jack Lambert and Bob John. Four inexperienced reserves were drafted into the side. They all performed badly and so did the regular members, with David Jack missing several opportunities to score. The tackling of the Walsall players, especially on Alex James and Cliff Bastin, also caused the team serious problems. As Bernard Joy pointed out: "They (Walsall) were aided by the narrow ground which was made more cramped by the encroachment of spectators up to the touchlines."
Fifteen minutes after the interval, Allsop headed in from a corner. Soon afterwards, Tommy Black gave away a penalty with a blatant foul on Bill Sheppard. Walsall scored from the spot and managed to hold out for a 2-0 win. It was the greatest giant-killing result in FA Cup history.
Gilbert Allsop died in 1992.