there are a number of reasons the email may be treated as spam:
If the from and the to address are the same
If the from address is not a valid email address
sometimes using email@example.com or firstname.lastname@example.org may trigger the email to be treated as spam.
does the header of the message include any info on why its regarded as spam, this would help us determine how to fix the issue, but without knowing why gmail thinks its spam, it's a bit like finding a needle in hay stack.