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.
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.