Firstly, I don't personally think that it is Gale Bertram, I just wanted to share my thoughts on why it COULD be him. 

The majority of theories on here that disagree with Gale Bertram being Red John state that it can't be him due to Rosalind Harker's description of Red John's hair.  

"Short straight hair" - it could be entirely plausible that Red John wore a wig to cover up his lack of hair. That would mean that it would be possible for the the balding suspects to be Red John. 

As well as this, (though even I find this doubtful) Rosalind may have been reciting the description that Red John told her to. She would have done anything for him, so maybe this is one of those things.