The baby's mother, Elizabeth Johnson, 23, hired Urias through a Craig's List ad to babysit Gabriel in a Texas hotel room while she was in the midst of a cross-country trip to escape a custody dispute with the boy's biological father.
"Elizabeth told me if anyone came knocking on the door not to answer it," Urias told ABC News.
Tempe, Ariz., police Friday released the photographs of Gabriel taken by Johnson in the same Texas hotel room in hopes it would help them find the missing baby.
Although Gabriel appears to be healthy and active, lying on a bed with toys and a pacifier, investigators fear that Gabriel is holding a medicine dropper in two of the photos .
"Elizabeth told me to give him a bottle in about an hour and then her exact words were to, 'Give him more medicine if he started crying to shut him up,'" Urias said.
"Gabriel wasn't acting like a normal baby," she said. "He was acting scared, he didn't want to play. He acted like he was sad."
The photographs were discovered on Johnson's camera following her arrest Dec. 29 in Miami.
In another disturbing claim, Urias said she found a large knife next to the bed in the motel room.
"It looked like a butcher type of knife and, you know, I thought that was weird because why would she have it next to her bed? It's something that would belong in the kitchen," Urias said.
The photographs were date-stamped Dec. 26, the last day Gabriel was seen.
Johnson is now in a Tempe, Ariz., jail and faces kidnapping, child abuse, custodial interference and conspiracy to commit custodial interference charges after she allegedly sent text messages to the boy's biological father, Logan McQueary, that she had killed their son and left his body in a dumpster.
Johnson later changed her story and said she gave the baby to a random couple she met in a San Antonio, Texas, park.
Mom Describes Mystery Couple Who Supposedly Have Her Baby
She described them as a white couple in their 30s. The man was tall with short black hair and the woman was 5-feet-3-inches with long blond hair.
Johnson said the couple looked trustworthy and brought their own car seat and that they said they could keep baby Gabriel without anyone noticing.
"Yes I'm sure he's alive," Johnson said in a jailhouse interview last week.
Investigators recently served a search warrant on the home of Jack and Tammi Smith, a couple who say they were in talks to adopt Gabriel before he disappeared and have since been named persons of interest.
Police believe the couple may be withholding information and took their phone records and laptop.
Tammi Smith still speaks to Johnson and met her in the Tempe, Ariz., jail last week.
"I looked in her eyes, I can tell you that baby is alive," Smith said.
"According to Elizabeth these people are going to try to hide this baby."
Gabriel's grandfather, Frank McQueary says that is all the more reason to be vigilant.
"That next-door neighbor may be buying stuff for a baby and they didn't have a baby before," he said. "Let somebody know."
Anyone with information about Gabriel Johnson is asked to call the Tempe Police Department at (480) 350-8311.
Baby Gabriel's Mother Denies Hurting Her Son
In a jailhouse interview with the Phoenix CBS affiliate, Johnson denied harming their son, saying she told McQueary she had killed Gabriel to get back at him.
"He had ruined my life, and he hurt me, and I wanted to hurt him," she said. "And that was the only thing I could say that would hurt him."
Johnson's grandfather, Bob Johnson, told ABC News that he doesn't believe his granddaughter's story that she simply left her child with an unknown couple.
No Clues in Johnson's Car
"I think that she has given the baby to somebody, and I think she knows who it is," he said. "I think it's in San Antonio in somebody else's house, and she just ain't given it up.
"She has an anger-management problem, and she's working on that," he said.
A search of Johnson's car -- found by the FBI nearly two weeks ago in San Antonio -- yielded no obvious clues or evidence of violence, police say.
Motherhood was never really Johnson's strong suit, her grandfather said.
"She discovered after eight months that there's more to this parenting than she wanted to get involved in," he said. "It was not her thing. She was into fashion."
ABC News' Sarah Netter contributed to this report.