My experience has taught me that gullibility is often equated with naivety and tends to be ridiculed and judged as immature. But gullibility has many advantages: you are open, you trust and you get involved in something that you don't really know or can control. The opposite of gullibility... I don't know. But I would say it would be hard-heartedness. Jadedness. Detachment. And in the long run, lifelessness. Maybe I'll be smiled at or disappointed or fooled if I'm gullible. But I also have the chance to experience something, to discover, to be surprised and to come into contact with something that is not accessible without my trust. So I think I would rather choose gullibility than lifeless hard-heartedness, despite the risks. And you? Perhaps you are being challenged with this decision in a concrete situation today.
I wish you an extraordinary day!