Gables are the pointed section of wall below the inverted "V" formed by two planes of the roof. The devices that I think you're are talking about are triangular shaped and fitted into the wall just below the roof. They are NOT gables, but are called Gable Vents.
If this is what your are talking about, then whether the contractor is telling you the truth or not about them just being "decorative" depends on whether or not there is ADEQUATE other ventilation method(s) provided in the roof construction.
All building codes REQUIRE adequate attic ventillation, and specify the total amount of opening required relative to the square footage of the attic area.
1. Tell your contractor this and then ask to show you the required ventillation devices. If he cannot, or will not, then stop dealing with him, don't argue with him, and contact your local building Inspection Department.
2. Give them the approximate square footage of the attic area, and ask them how many square inches, or square feet of attic ventillation is required, AND the methods of venting allowed.
3. Then when you understand the process, go back to the house when the contractor is not there, and look for the attic vents.
There are several methods of venting. Gable vents, Ridge vents which attach along the Ridge [very top, center of the roof, and are often concealed by being covered with shingles, leaving a thin gap between the vent and the roof below], Slotted and screened vents all along the underside, and length of the Eave overhang, many small round vents with [2 to 3 inch diameter] louvres [slots] in them.
4. IF you cannot find any attic ventillation method, then go bact to the Building Inspection office, and ask for an Inspector to come check it for you. If proper ventillation has been provided, the s/he can point it out to you, and IF there is no ventillation, or it's inadequate, then s/he will "make" the contractor do it right.
This step 4 is going to piss-off your contractor, but if it wasn't done properly, then you could have serious moisture, and possibly MOLD problems "down the road."
IF there is "other," adequate ventillation for the attic, then your contractor is right, and there is no need for Gable vents, and cosmetic simulated "fakes" could be applied IF you wanted them for asthetic reasons.