It does not necessarily make you stupid or gullible; but it's certainly possible that you're stupid or gullible. If someone put forth a sincere, passionate defense of the "flat earth" theory, it would not automatically mean that person was stupid or gullible, but the rest of us would certainly not rule out that conclusion.
I think people are more making fun of you than attacking you. If you are going to voice the views that you do, you've got to be prepared for a little derision. As much as Free Republic tries to avoid (and delete) profanity-laced ad hominem attacks, that does not mean it's safe for the thin-skinned.