Wales coach Gatland delayed naming his side to face Ireland in a bid to give centre Roberts and fly-half Priestland as much time as possible to prove their fitness following knee injuries.
Both were named on Friday in the team to face Ireland at the Aviva Stadium in their RBS 6 Nations opener, although flanker Dan Lydiate misses out with an ankle injury.
Priestland's return, following a tremendous debut World Cup in New Zealand, is a huge boost for Gatland while British and Irish Lion Roberts has long been considered one of Wales' best attacking threats.
And while expectation has reached fever pitch in Wales after Gatland led the country to the World Cup semi-final last year, the Kiwi claims his decision was based purely on medical advice.
"We are happy that Rhys and Jamie have come through. Unfortunately for Dan, he is not quite ready," Gatland said.
"He might be ready next weekend [against Scotland], but it is an opportunity for Ryan Jones at six.
"I can only take it on what the medical staff say, and they say they are fit. So if they tell me they are fit and we think they are good enough to be selected, then we are going to do that.
"We had a report from the medics yesterday saying they were confident they would be right for Sunday.
"Jamie had a few sessions during our training camp in Poland and Rhys had a couple. They haven't taken a full part, but we are quite happy with the amount of rugby they have got under their belts for this game."