That's right, Gouken is officially in Street Fighter IV.
Diagonal single-hand hadoukens, Shin-Shoryuken, hell yeah.
Basically if you want to fight him yourself just head down to an arcade with SFIV and play through while doing the following:
1. Do not use any continues
2. Do not lose the first round in any fight
3. Get a certain number of perfects depending on what the machine is set to.
(Set to 1 round: Must get a perfect in at least one match.
Set to best of 3: Must get 2 perfects
Set to best of 5: Must get 3 perfects
Set to best of 7: Must get 4 perfects)
Once you beat Seth, Gouken will appear to teach you a lesson in PAIN.
I may not be a very good SFIV player but damn I'm gonna keep training, maybe one day I'll be good enough to face up against the old fella.... but probably not anytime soon :P
Found on The Streetfighter Blog
and The SRK forums