What does this mean? For starters: everyone's gut reaction is their own. Yours doesn't have to agree with mine, but if it does, then that's more reason to heed. To me (probably because I haven't learned enough yet), it doesn't mean you must act on it; it just means you retain it, keep it in the hip pocket, or like a tip sheet for future use, for those moments when you will inevitably (due to human nature) go against it.
We all do this -- we all go against it. They wouldn't be called "first impressions" if there were no second ones and third ones... Maya Angelou has a famous phrase, "When someone shows you who they are, believe them." Lots of people think this phrase can be meant as a warning system, and I'm sure that's its main intention.
On the opposite side of the same coin though, is that it's also an "it's OK" system too. If your first candid impression of someone is as lovely and tender and sweet, then go with that -- even if they show you something different later -- because you know it is there. I wouldn't be married to the man I am if I ignored his tenderness the first time I ever met him (that said, he has never shown me another side). So if your nice person is snarly, it's probably because something's wrong. Then, listen to your gut to help root out the problem.
Going against our gut, and going for the familiar can create problems and waste so much time: I would have never dated as much as I had and I would have never learned the lessons I learned if I had always gone with my gut (I would have saved myself a lot of heartache and woe, but hey, I'm not Spock). This is part of life. We buy the wrong car; we talk to the crazed person we saw scream at traffic; we take the job and it doesn't work out; we gloss over the tone when we heard someone say something ... we overcompensate, we rationalize.
That's OK. Don't judge yourself, but DO know that it won't stop -- these face palms, the "not again" moments until you stop. Until you see the light at the moment you're really meant to. That when all the data is lined up, and you've learned all you need to know -- about the book, or the song, or the person you saw screaming at traffic or the job interview you had -- that your Gut Instinct will be there, waiting and saying, "Welcome back, normal-thinking self, that person we spent so much time in therapy trying to find, we missed you."
So then the trick is after we finally Get It, to not beat ourselves up too much for not Getting It in the first place. It's OK if you stumble and ignore your Gut. There's a phrase "against our better nature" that comes to mind. I personally dislike the use of "better" because it is judgmental; it implies that we should know "better." Sometimes, as in matters of the heart, we simply Don't Know Better. Until we do.
Just don't beat yourself up while you're learning -- and more importantly: don't let whatever you're learning about beat you up either; don't ignore the flags, apparently they are always waving.
And then when we do figure it out... Hot diggity, Woo-hoo and Allelujia, it's a good thing and Lesson is Learned. You have FINALLY Gone With Your Gut. Now it's time to Repeat! The wisdom from the lesson ... NOT the lesson.