NT Chief Minister Natasha Fyles announced army veteran and father-of-four Brent Potter would be Labor's candidate for the seat of Fannie Bay, formerly held by Mr Gunner.
Mr Gunner revealed he would step down as chief minister in May, and resigned as the local member on Wednesday.
Ms Fyles' announcement of Mr Potter as the party's candidate on Friday did not mention his recent employment in her office.
Mr Potter's LinkedIn profile had been removed from public access by Saturday, but Google cached results from the site list him as Ms Fyles' senior adviser.
"I know he will work hard for the Territory and the electorate of Fannie Bay," Ms Fyles said in a Facebook post announcing Mr Potter as the candidate.
"Brent knows what it takes to serve the community - and that's where his passions lie," the chief minister said.
Mr Potter said serving the community is what he does best.
"There is no greater feeling then being able to serve, deliver and lead for the community and those around me," Mr Potter said.
He said the short campaign before the August 20 by-election would nonetheless be a strong one.
The Country Liberal Party is yet to announce its candidates, with nominations set to close on Sunday.
"The CLP has been enthusiastically approached by various community leaders, local professionals, and small business owners canvassing interest in running as our candidate," CLP president Fiona Darcy said on Wednesday.
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