She said the woman, a customer, was waiting in a queue and kept looking over, glaring and making "tsk tsk" sounds.
But she wasn't phased.
"It's her problem not mine.
"I was really surprised because I think compared to my grandmother's day, society is a lot more open minded."
Ms Gray said it gets easier once you know what to expect, including a little bit of pain and perseverance.
"With my first, it was a little harder just getting the latch, also trying to do it discreetly in public takes a bit of practice.
She advises new mums to keep on trying because it is a good bonding opportunity.
"Know that it gets better, it can be hard at the beginning, but it's really awesome."
Ms Gray decided to breastfeed because it's more convenient than sterilising bottles and having to buy formula.
"It's easier to breastfeed because it's there, it's really accessible, I know society encourages breast feeding but I think it's hard for mums both ways, just do what suits you."
She said was pretty excited to go to the event as she's wanted to go before.
New mum Katie Laporte, 37, participated last year and said it had a great social atmosphere.
She breastfed her son Tommy until he was 10 months old and chose it over formula partly because it was a lot easier and felt natural.
"It's easy, I never had to take bits and pieces to make up a bottle."
There are also health benefits, she said.
"I felt like breast milk agreed with him, he didn't have any skin problems, he was really healthy."
Ms Laporte said it was difficult for first three weeks.
"It takes a little bit of getting used to."
She advises new mums not to put too much pressure on themselves to do everything and use any support from family and friends.
"Just relax and enjoy the new experience of having a newborn."
She said she never had any issues in public and there are lots of clothing options to be discreet.
The Big Latch On takes place on Friday, August 1 at the Carterton Events Centre.
Doors open at 9.30am and the latch count is at 10.30am.
Breastfeeding Wairarapa Coordinator Clare McLennan-Kissel said they were aiming for bigger numbers than last year when 48 women latched on.
By bringing the event to Carterton, they are hoping to get more South Wairarapa mums and families participating.
To mark the event's decade anniversary, women can also take part on social media by posting a breastfeeding selfie with the hashtag #biglatchonNZ.
