Mr Gillott, from the Witterings, West Sussex, said: ‘The photograph in his obituary was an exact match for the painting.
‘I defy anyone to say it is not the same person - I would say the photograph has been used to create the memorial portrait.’
Records show the young officer was the only son Lieutenant-Colonel John Francis Girardot and his wife Mary.
His widowed mother was born in Cardigan, West Wales, and lived in Carmarthenshire after the death of her only son.
Staff at the Carmarthen County Museum, in West Wales, are delighted they can now put a name to his face and have begun telling the lost story of his sacrifice to local school children.
The painting 'Portrait of an Unknown First World War Officer' has now been renamed 'Second Lieutenant Paul Chancourt Girardot (1895-1914)' - and the story of his sacrifice revealed to a new generation.
The museum had no record of when the portrait came into their possession but the soldier's identity was unknown when they re-catalogued their art collection in the 1970s.
Museum spokeswoman Debbie Williams said: ‘We are delighted this discovery has been made - it is wonderful to finally find out who he is.
‘It is a tragic story but it is always nice when a piece of the jigsaw is put back in place.
‘The portrait has gone back on display with the new name and information about his service added.
‘We have a lot of school children coming through our doors and in this First World War centenary year it is wonderful his service will be remembered by a new generation.’