Robert Churchyard, 52, denied manslaughter by gross negligence at Norwich Crown Court but was found guilty by a jury after an hour of deliberation.
He was bailed until 30 October, when he is due to be sentenced along with the company. Judge Stephen Holt told Churchyard: “This is a very serious matter and a custodial sentence seems highly likely in this case.”
Churchyard fitted the gate, which weighed about a third of a tonne, at Mrs Lunn’s home in Blofield Heath near Norwich in March 2013.
The court heard the gate was designed to be operated both automatically and manually, but fell because safety-stop devices had not been fitted to prevent it sliding off a steel track on the ground when opened by hand.
When the remote-controlled automatic mechanism failed, Mrs Lunn pulled the gate open and it fell on her.
At a separate trial last year Churchyard, of Turner Road, Norwich, was found guilty of breaching health and safety rules.
Churchyard, who chose not to give evidence during the trial, worked for Automated Garage Doors and Gates, in Burnet Road, Norwich.
The company, which had admitted three counts of failing to comply with a requirement under the Supply of Machinery (Safety) regulations at an earlier hearing, will be sentenced alongside Churchyard next month.