Elementals can see, have no immunity to gaze attacks, and no immunity to petrification. As such, yes, they are susceptible to the effects of the petrifying gaze.
See the Elemental Type rules:
An elemental is a being composed of one of the four classical elements: air, earth, fire, or water.
An elemental has the following features.
- 8-sided Hit Dice.
- Base attack bonus equal to ¾ total Hit Dice (as cleric).
- Good saves depend on the element: Fortitude (earth, water) or Reflex (air, fire).
- Skill points equal to (2 + Int modifier, minimum 1) per Hit Die, with quadruple skill points for the first Hit Die.
An elemental possesses the following traits (unless otherwise noted in a creature’s entry).
- Darkvision out to 60 feet.
- Immunity to poison, sleep effects, paralysis, and stunning.
- Not subject to critical hits or flanking.
- Unlike most other living creatures, an elemental does not have a dual nature—its soul and body form one unit. When an elemental is slain, no soul is set loose. Spells that restore souls to their bodies, such as raise dead, reincarnate, and resurrection, don’t work on an elemental. It takes a different magical effect, such as limited wish, wish, miracle, or true resurrection, to restore it to life.
- Proficient with natural weapons only, unless generally humanoid in form, in which case proficient with all simple weapons and any weapons mentioned in its entry.
- Proficient with whatever type of armor (light, medium, or heavy) that it is described as wearing, as well as all lighter types. Elementals not indicated as wearing armor are not proficient with armor. Elementals are proficient with shields if they are proficient with any form of armor.
- Elementals do not eat, sleep, or breathe.
None of this says that elementals do not see, none of it mentions immunity to gaze attacks or petrification. So they’re vulnerable to all of these things. A fairly-low Knowledge (the planes) check could have meant the Cleric was aware of this before he summoned the creature.