In Pokemon Gold there are a number of options.The most viable is using a Growlithe.Who can learn Bite,a dark type attack as well as Dig which in Generation II works on Haunter and Gengar because they don't have the Levitate ability.
Another option is to train a Psychic Pokemon like Drowzee or Kadabra.I personally would use Kadabra because you need to be able to hit his Gengar first because it knows Shadowball which will devistate your fragile Kadabra.
Another option is to just overlevel your Pokemon.If you chose Totodile then that would be a good one because he can learn bite he also evolves into his final form relatively early which greatly boosts your stats.
Good Luck
Professor Juniper gives it to you when you go to beat the last Gym leader.
the 7th gym leader is in sunyshore city and he is the user of electric type pokemon all you gotta do is beat him and all the trainers and he will give you the badge just like the others
He is the last leader.
Their is no last gym leader the show will never end. well in the show right now? Volkner is going to be the last gym leader.
Veridian city
beat the last gym leader on soul silver in johto
try the improved elite four or try to beat red on mount silver.
you battle them in the dance center after you defeat Claire, the last gym leader in johto
You need to beat Pryce the second last gym leader. He uses ice Pokemon so bring fire or steel Pokemon.
get stronger then you are before
you beat the last gym leader in johnto
you have to beat the last gym leader
train your pokemon more.
The last gym is in viridian city
Blue, the last gym leader in kanto region
in mohgaony town
you should have a lvl 50 water type