It's not Gale Bertram, or Bret Stiles, but most likely is a tag team made up of:
Kessie Dunninger
Hollis Dunninger
Mandy Schultz
Gangs recruited by these three

Let's look at why these three, or at least, Kessie and Hollis anyway.
Kessie Dunninger from "Redemption".
Why?
1) She has red-hair, so could well be the 'Red John' - the 'Red' part anyway.
2) She may be tag-teaming to do the Red John, she being the thinker and the 'John' being the doer [although who the John is, well, open to question)
3) There has been this theory going round on a few websites discussing The Mentalist. It makes enough sense in context.

But who could Red John's accomplices be:
1) Her dad Hollis, the 'John' of the partnership; this theory was posted on a showbiz site in the UK. 'John', if you think about it... 'John' being slang for an everyman, an average man.
2) Mandy Schultz (who apparently made a cameo during one of the episodes which had Lorelei in it) - helping Kessie plan the murder? (that theory was posted on a website few months back).
3) A small gang of people that are recruited to carry out these murders recruited by the Red John team.

It would not surprise me if Red John was a non-existent person, and just a pseudonym for a group of people, a 'house name' for a murderer if you like.

Theories about it being the CBI / FBI etc. could be debunked based on this line of reasoning; I'm surprised that the characters of Kessie and Hollis Dunninger and Mandy Schultz haven't been added to the accuse list on the site!

The motive, though, is a little fuzzier, but British fans continue to debate why.

I would be surprised to see what your comments are on this theory; it does make a bit of sense, and is widely popular with British viewers.
I am a UK viewer of the show, just for the record.