Obviously all tanks will be able to MT in some capacity. The example used is PLD. PLD is an OT and is seen by the community as an OT due to the way stances work and because it has a lower enmity generation on pull (also because of party support like cover, intervention and clemency). Having said that, PLD can MT if it needs to, it just isn't as good as having a DRK or WAR in that slots. I highly expect GUN to be the same, so all 4 tanks can do both, but they either excel at MT or OT.

Also, if OT tanks couldn't MT, the Devs would have to remove tank swaps from the game, consider that.