were do you find the 6th Gymleader in Ruby

you can find feebas in route 144

how do you evolve feebas

Go to bulbapedia and you will see information about pokemon

feebas takes an assload of patience to find! on route 144 there are only six squares of
water (out of around 400) that you can fish in to find feebas. they are completely random,
and they will change only if you go change the trendy phrase in dewford. just fish around
route 144 to get a feel for where feebas might be. also it wont appear right away if you
do find one of the squares.

to evolve feebas you have to give it a certain type/amount of pokeblocs, not sure what
kind, i think to get its beauty up all the way, then it will evolve to milotic