I would not worry about her sabotaging your ds' application for a gifted program. The teacher may think that "lots of kids are good guessers".... but she can't *successfully* argue that a valid IQ test such as the WISC/WJ-III Test of Cognitive Abilitiies/SB/etc aren't valid tests or that they can be "guessed" into a high score. She also can't argue a professional's credentials aren't valid. We've been caught in similar situations where school staff tried to claim IQ/etc were meaningless and our ds was "in the middle" - and the very best tactic that worked for us in those situations was to simply state the facts calmly, over and over again until the school staff moved past the argument. The facts - understand your bell curve and be able to explain your ds' score relative to it, and know the credentials of the professional who administered the test. One of my favorite responses that was quick to shut down these types of arguments from teachers/etc was to reply "Are you telling me you disagree with the opinion of a well-respected licensed neuropsychologist?"
Fortunately, this school year is nearing completion (in the US)... so even if things never get better with this teacher, she will be out of your life soon.
Re the gifted program, it sounds like they are looking for more than one piece of data to determine eligiblity; I suspect that a qualifying WISC score from a reputable tester is going to trump one negative teacher's input. I also have seen patterns in teachers at our children's schools - a teacher who is telling you the things she's told you about your ds is also quite possibly saying the same things to other parents who are pursuing something more for their children... and she might not be making referrals to the gifted program at the same rate as another teacher who might really want to see a lot of her students get into the program.. and the people who are making admissions decisions for the program may discount her opinion simply based on past experiences with her recommendations.
The very worst thing that could happen re the gifted program is that your ds could be denied admission, and if that happens, you ask for the specifics of why, and you appeal.
Best wishes,
polarbear
