Ryan Leaf Offers To Help Josh Gordon ... 'I'm Here For Him'

Ryan Leaf says he's glad Josh Gordon is stepping away from football to focus on what's really important -- his mental health -- and the ex-QB says he's willing to help if the WR needs him.

Leaf has fought a very public battle with mental illness ... and warns Gordon -- who left the Pats on Thursday to focus on his mental health -- the fight he's facing is a very difficult one to win.

"This will be the hardest thing he ever does," Leaf tells TMZ Sports.

"Making it to the NFL for him was easy in comparison to what he has to battle and what he goes through on a daily basis."

But, Leaf -- who's sobered up and now hosts a sports talk show on SiriusXM -- says he's down to help ... and has actually…