A Grass Type in Johto is more a curse then it is a blessing. Being resisted and beaten by the 1st and 2nd Gyms. They prove some worth on the 3rd Gym but then resisted on the 4th. The 5th Gym has a Water Type and most Grass Types are part Poison so thats the BEST place they shine. Then they suck again at the 6th and 7th Gym, though they can do some damage to the 6th Gym's Steelix. The 8th Gym is the neutral, being hit by Dragonair's Fire Blast isn't to welcoming BUT it does resist STAB Hydro Pump from Kindra.
Then the E4 has Bug, Flying, Poison, Fire and Pokemon with the said moves to boot. The only time the would do ANYTHING is on Slowbro and on Onix, though they could be some good fodder on Lance (regard the E4 comment if you use Jumpluff, because it trolls the E4 completely)
It gets better at Kanto resisting Lt.Surges STABs but sucks at Sabrina (if it has a Poison secondary type, if not its Neutral) Janine will make you bite the dust, Blaine to. Though it has superior typing against Misty and Brock. Erika resists you, but can't do that much to you either. Green/Blue (forgot the name) overkills you with EVERYTHING bar Rhydon.
Red has Blizzard abuse so a Grass type is only useful against Pikachu. And could be useful against his Blastoise and Lapras if you work around the Hail.
Grass types arn't usefull at all but if you WANTED to use one, use Vileplume, Jumpluff or Meganium.
Vileplume gets Grass, Poison, Fighting coverage, Jumpluff has Status abuse and Meganium is a complete wall even more so with Screen support.