Johnson said the two 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.
Investigators recently served a search warrant on the home of Jack and Tammi Smith, a couple who said they were in talks to adopt Gabriel before he disappeared and have since been named persons of interest.
Police believe the two may be withholding information and took their phone records and laptop.
Tammi Smith still speaks to Johnson and recently met her in Tempe.
"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, said 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.
ABC News' Sarah Netter, Angela Ellis and Kate McCarthy contributed to this report.