The Cambridge history student knew the "significance" of the monument but did not realise he was dangling from a war memorial, the Court of Appeal was told.
Gilmour, of Billingshurst, West Sussex, was jailed for 16 months for violent disorder in July.
The 21-year-old is appealing against his sentence.
Judges heard Gilmour had taken LSD and valium before joining the central London protests against increased tuition fees last December.
He was seen hanging from a Union flag on the Cenotaph and leaping on to the bonnet of a Jaguar - part of a royal convoy.
Gilmour was previously found by a judge at Kingston-upon-Thames Crown Court to have thrown a rubbish bin at the vehicle.
His lawyers are also challenging this ruling as part of the appeal.
The crown court judge had accepted that the incident at the Cenotaph did not form part of the violent disorder, but described it as "outrageous and deeply offensive behaviour".
During his several-hour-long rampage, Gilmour was also found to have kicked the window of Topshop's flagship store on Oxford Street.
He ended the day in possession of the leg of a mannequin.
Gilmour's defence QC, David Spens, said his client did not know what the Cenotaph looked like and, as he had lived "without a television for 10 years", this was "not actually as surprising as might have been thought".
The court heard he had turned to drink and drugs after being rejected by his biological father, the writer Heathcote Williams.
Mr Spens said he had "successfully reformed and rehabilitated himself" and addressed the "underlying drug and alcohol problems".
He added: "My submission is that this sentence was unduly harsh, unnecessarily so."
The court has reserved judgement pending other Court of Appeal rulings relating to the August riots, which may have a bearing on Gilmour's case.

It began in November but 20 cases have been notified in the last week and the total is more than for the whole of Wales in the last three years.
So far, 32 secondary schools, primary schools and nurseries are affected.
Parents are being urged to make sure their children receive the measles, mumps and rubella (MMR) vaccination.
The outbreak only affects the Abertawe Bro Morgannwg University Health Board area and eclipses the number of cases across the whole of Wales since 2010.
Last year's figure of confirmed cases for Wales stood at 116, while there were just 19 in 2011 and eight the year before, according to Public Health Wales.
There were 159 cases in 2009, still lower than the current outbreak in south west Wales.
Dr Marion Lyons, director of health protection for Public Health Wales (PHW), said: "We continue to be concerned at the number of cases of measles we are seeing in the Swansea and Neath Port Talbot areas.
"We cannot emphasise enough that measles is an illness that can kill, or leave patients with permanent complications including severe brain damage, and the only protection is two doses of the MMR vaccination."
She added that people most at risk of catching measles are children of school age who have not had two doses of MMR.
Children should receive the first dose of the vaccine at 12 to 13 months of age and the second at three years and four months of age
PHW estimates there are more than 8,500 school children at risk of measles in the Abertawe Bro Morgannwg area.
Many people who catch measles will have a fever, cough, red eyes, and blocked nose and feel generally unwell.
The blotchy rash appears a few days later beginning on the face and spreading downwards to the rest of the body over several days.

A lady was knocked over during the theft in Market Street, Limavady, which happened at about 15:00 BST on Friday. She suffered non life-threatening injuries.
Police have said the officers were nearby when the incident occurred and a 20-year-old man was detained.
Supt John Magill commended the off-duty officers.
"I feel that this is a perfect example of police officers continuing to keep people safe, whether they are on or off duty at the time."
Charlie Gilmour, the son of Pink Floyd guitarist David Gilmour, was unaware he was swinging from the Cenotaph during student protests, a court has heard.
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An outbreak of 189 cases of measles has been reported in Swansea, Neath and Port Talbot, Public Health Wales says.
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Two off-duty police officers have arrested a man after a burglary at a shop in County Londonderry.