"I did not expect to win, so the events at Cardiff City Hall were very hard to take in," she recalls. "I felt absolutely numb when my name was read out and it took a few seconds to realise they were actually talking about me."
Head of Ysgol Penybryn in Tywyn, on the west Wales coast, for the past five years, Gillian's success is all the more remarkable considering the relative lack of Welsh spoken in the area.
Born and raised in a Welsh family in Aberystwyth, she began her teaching career in the staunchly Welsh stronghold of Bala, only to discover things were rather different in Tywyn.
"We found that just 7 per cent of our children actually speak Welsh at home," she says. "Tywyn is a seaside resort which has changed a lot over the years.
"Obviously, there are pockets of Tywyn that are very Welsh, but because a lot of people have moved in from places like the Midlands, many of the children who come to us have no background in the language at all."
Gillian and her hard-working staff have risen to this challenge by getting their pupils interested in Welsh culture, history and their actual locality as well as the language itself.
"It's not just a case of getting them to speak Welsh but also getting them to consider what it means to be Welsh," she explains. "We get them to consider the history of this area as well as Wales's place in the world."
A mother in her forties, who breeds horses at her home near Machynlleth in her spare time, Gillian maintains that the secret of her success is simple: get would-be Welsh speakers young.
"During their first year here, children are totally immersed in Welsh and the results are there for all to see," she says. "A lot of hard work is involved, but it's wonderful to see those same children totally bilingual by the time they leave us for secondary school."
Like many Plato winners, Gillian sees her award as fitting recognition for the unstinting efforts of a dedicated team rather than a lone individual.
"It's not just me - we have an excellent group of parents and governors without whom the whole thing just wouldn't work," she says. "We are also lucky to have the support of the local education authority, which has a lot of input into the school."