And he has not ruled out Gavin Henson featuring in the World Cup selection shake-up, despite omitting both players from his 28-man RBS 6 Nations squad.
Williams, 35, is just two caps short of reaching 100 for Wales, having made his Test debut in 1996.
But Gatland has preferred Sam Warburton and Josh Turnbull as openside flanker options for the Six Nations campaign, which Wales begin against England in Cardiff on February 4.
Centre Henson, meanwhile, has started his comeback with Saracens after 20 months in the rugby wilderness, although he is currently sidelined because of a calf muscle injury.
Asked if Williams might still have opportunities with Wales, head coach Gatland said: “Absolutely, there is always a chance for Martyn.
“I spoke to Martyn on the phone before the squad was announced. We knew what a quality player he is, and has been, a great servant to Wales.
“There are still one or two players that might be added to the squad later on, someone like George North, who is coming back from injury, and with Gavin, it has been stated he needs to play some rugby.
“He needs to get some games under his belt. I thought when he went on for Saracens against Leinster (eight days ago) before he pulled his calf muscle, I thought he showed a couple of nice touches.
“It would be good to see him get one or two more games under his belt, and he may come into contention, but not until he does that.”
As for his prospective team selection to face England, Gatland added: “We are pretty close to what we think is a team that will take on England.
“I don’t think there are going to be too many surprises in the England side. I think they are fairly predictable.
“We are quite clear of what team we think England will pick, and looking at our squad, they can fairly comfortably get close to picking our team as well.
“We may see a couple of contrasting styles on Friday week. We expect England to really take us on at scrummaging and drive a lot of lineouts.
“We’ve got to match them up-front, but we also want to play some rugby.”