What do i do when i've defeated Red in puso Gold?

well u shud go to professer oak he will let u choose wich starter u want like charmander squirtel or bulbasauar and then when u choose one your done go train the pokemon u choosed and then go to saffron city in the silph co. place talk to a guy he will give u one of the starters of heon i think if u dont find himm go to the club house in vermilion city then get out of the house and the guy il talk to u the go to safron city again and look for him

well, if you haven't already, you can try and catch the legendary pokemon. believe it or not, there are a lot of them. that includes the ones from the other regions(or most of them, anyway). i'll listahan the ones i know you can catch. You can catch. Ho'oh, Lugia, suicune, entei, raikou, articuno, moltres, zapdos, mewtwo, latias, and latios. You can also get kyogre only on heartgold, and groudon on soulsilver. you can also get raquaza if you trade either groudon or kyogre from the other game (has to be from other game). you have to look it up on youtube or something to find out where to get 'em.

Well, you CAN'T go to every region. On thing you can do is go to the Silph Co. and get a Hoenn starter, and one a Kanto starter from Prof. Oak.
The Battle Frontier is now available, and the Gym Leaders/ Elite 4 can be re-battled. Here's where all GLs can be re-battled. link

And you can always catch 'em all!

Sorry if this doesn't help but all I know is once you defeat Red, go to Professor Oak's lab and he will give you a choice of Charmander, Squirtle or Bulbasaur. I picked Charmander. Sorry if this didn't help. But I do recall going into a cave going to the new Safari Zone west from Cianwood City. On the floor are Kyogre patterns. Wait, sorry. I just went to Silph Co. and I just got a Treeko, but there are Kyogre patterns in the tower.

well i dont think you can go to all four regiens mayb u can try completing pokedex? i know this doesnt help...=="

try cating some legendaries or get all the medalya in the pokeathon thing