Racing Metro centre Roberts, 28, is understood to have rejected Cardiff Blues and agreed to join Quins after the 2015 Rugby World Cup.
Head coach Gatland has previously stated he would prefer Welsh internationals to play in Wales.
"There's not a hard and fast rule as there is with England," Gatland told BBC Wales Sport.
"There are exceptions where players have been playing outside of Wales and there are one or two wildcards we could pick.
"They do play outside of Wales but they could be selected, but I think the long-term goal is to have a policy where, ideally, we pick players who are playing in Wales."
English players who move to play abroad are currently not considered for national team selection.
To complete the move to Quins, Roberts must first be released by French Top 14 club Racing Metro, where he has a contract until June 2016.
Under the so-called 'Gatland's Law', Wales-based players are likely to be favoured for Test selection over those who choose to play elsewhere.
In August 2014, the Welsh Rugby Union and Wales' four professional regions signed a £60m, six-year deal that settled a long-running dispute over the sport's future.
That deal included rules that mean players based outside Wales could also be overlooked in favour of home-based talent, with coach Gatland expected to tighten selection policy.
The selection policy ruled that no player based overseas should represent Wales, although Gatland is able to make exceptions.
However, that only applied to players who moved before the deal was agreed, and so at the time would not have applied to Roberts, who joined Racing in June 2013.
A move to Harlequins now would bring the centre into the category of players who could be overlooked for selection, though Gatland remains pragmatic on the matter.
"The ideal for me is you want players playing in Wales," Gatland said at an event where tickets went on sale for Wales' World Cup warm-up matches.
"It's important we work closely with the regions to make them as successful as possible.
"That will encourage players to stay and encourage players to come back to Wales."
Wales host Ireland on Saturday, 8 August, visit Dublin on Saturday, 29 August, and then welcome Italy to the Millennium Stadium on Saturday, 5 September.
Warren Gatland insists Jamie Roberts' expected move to Harlequins will not preclude him from Wales selection.