I think you do have to be very open-minded to accept astrology. In a sense, often more open-minded people are more gullible because they are willing to accept much of what is presented to them. Whether they take it on board as complete and utter fact is a different story, but by nature they probably will be at least willing to hear different ideas about the world, leaving them vulnerable to being taken advantage of - IF they are too trusting.
I think the only valid point concerning astrology's validity is the one that demonstrates to us that sometimes, you can quite easily attribute certain qualities to someone that are very broad and far too vague that really could be applicable to anyone. When you are not taking into account the whole chart and every little detail that goes into it, this is all too possible, if not likely. I have seen this happen a lot, and I try to catch myself before I do that. People who are interested in astrology but don't know much about it can be rather gullible if they take those kinds of comments at face value without looking at astrology in more detail. To accept anything without putting thought into it is gullible, whether what is being presented is true or not. While some of these people are gullible, others are just having fun or are hoping for that sudden sort of enlightenment, you know, where the spooky old psychic tells you that you will marry this or that dude and it actually comes true. Attitudes are all very influenced by pop culture and the media, so a mild belief in astrology does not always come down to an inherent gullibility (though it can), as this kind of approach is less thought out and will not lead to upset and confusion if astrology is somehow proved wrong indefinitely.
When it comes down to it, however, I think most people are pretty gullible, astrology believers or not. Something is shown to them and for some reason it appeals to them - with religion, with astrology, with any kind of belief or group. People like to join up and be a part of a group, and often are more than happy to sacrifice their own personal understanding for the security and comfort of belonging to a belief system of some sort. It's the way we are. We are social beings, and groups and the sense of belonging they bring are very important to our psyche.
So to answer your question: I think some are gullible, some are not. Those that are not are, I feel, those who do plenty of research and look very clearly and in a non-biased way at astrology, eventually coming to a very personal understanding as to what astrology has to offer.
Being interested in astrology is different than practicing it religiously. I like astrology mainly for self discovery purposes. I try to look at the similarities and differences between people and how they correlate to their astrological profile. Astrology produces stereotypes, which I've only noticed on Y!A, but its the signal that you are taking astrology too seriously. Its for our entertainment and spiritual imagination. Not necessarily to be used as a rulebook for our lives.
Taking interest is more of being open minded than it is "gullible".
A lot of them are and a lot of them aren't, astrology is something i do for some brief entertainment and i find it incredibly interesting at the same time, and through it came more self-understanding, and i am not a gullible person, as ive grown over the years ive been able to see through all the bul*shi* and can tell if a person's lying just by looking at them and their mannerisms.. i follow my intuition a lot and it's always right, just be a confident person and observant when somebody is talking, stand up for yourself and have some dignity and you'll never be taken advantage of, it's up to you to be taken advantage of in the first place.