Ranged > melee.
Sniper > other ranged.
This is true, but only in PvE. Assuming you want to keep a semblance of balance in your ops team (taking equal melee and ranged DPS into an operation), the versatility of the Gunslinger/Sniper in my opinion trumps both Sages and Commandos. While it is true that Gunslinger damage lags behind the DPS of melee classes, this can be said for all ranged DPS really and I think they should be evaluated separately.
Gunslinger burst spec (Sharpshooter) does similar or slightly higher damage than comparable ranged burst specs (Gunnery, Telekinetics - the burstiness of which is up for debate). Gunslinger sustained DoT spec (hybrid) does similar or slightly more damage than comparable ranged sustained DoT specs (Assault Specialist, Balance). However, I believe the difference in terms of raw damage numbers is small enough for player skill to be the major determining factor.
Where Gunslingers do have a noticable advantage over other classes is the fields of mobility, utility and defensive abilities:
- Hightail It provides mobility similar to Force Speed for Sages and flat out trumps Hold the Line. They have the same cooldown I think (20 seconds) and while the movement can be executed with one button, it does not allow for directional change. Sharpshooter Gunslingers, however, have further access to passive speed boosts (e.g. after Hunker Down ends). IMO, this makes SS one of the most mobile specs in the game.
- Hunker Down makes the Gunslinger immune to physics and lasts twice as long as Hold the Line in Gunnery spec. Snipers have a definite advantage here over both Commandos and Sages, especially considering that Force Barrier's cooldown is 3 minutes and forces Sages to completely stop any previous activity.
- While it is not % based, the cooldown of Defense Screen is essentially nothing (20 seconds), as opposed to Reactive Shield's 2 minutes. This allows Gunslingers to use it very often and requires far less conscious timing. Activating it during the fight is not a strategic decision (like Reactive Shield or Force Barrier). While it absorbs less, you can use it more often, which should equal to more damage being mitigated in the long run. Dodge can act as a self-cleanse and fully mitigated melee/ranged damage. Gunslingers have access to further damage reducing talents in Sharpshooter tree (Holed Up, Ballistic Dampeners). Because Crouch is not on the GCD, the perpetual crouch/uncrouch every 6 seconds can indefinitely maintain Ballistic Dampeners for a continuous 15% damage mitigation. Furthermore, the strategic cooldown of Scrambling Field reduces party wide damage by 20% for 20 seconds. Lastly, the careful use of Hightail It can also act as a defensive ability, negating all damage for the duration.
When you take all of this into account, I'd say that Gunslingers are more desirable than either Sages or Commandos but not primarily because of their damage. They have very good effective DPS but they bring considerably more mobility, utility and defensive abilities to the table. The only notable disadvantage is that they don't have on demand, instant heals (like Force Mend or Kolto Bomb) and they cannot cleanse themselves as frequently as Sages and Commandos can. The former issue is almost non-existent if the operations group has competent healers, while the latter only applies on one boss (Nefra).