What is an adjective that means a person that easily trusts others?
Solution 1:
Credulous is a good alternative to gullible.
Solution 2:
Not to sound too obvious, but what is wrong with "trusting"?
John was very trusting, some might even say gullible.
Your question is a little unclear because gullible doesn't just mean "easily trusts others", it really means "easily deceived," or "easily taken in." For sure, a gullible person might be trusting, but I don't think all trusting people are necessarily gullible.