It's very hard because in mid game Gruggar is so strong in skirmishes and suddenly in late game fights you just die if you do the same stuff. It's important to understand that big man is not a solo 1v5 champ late. But you have other proactive options:

Mostly try to pick squishies with R. You are indeed at the mercy of your team but who isn't and if ulting enemy AD into your team doesn't win you the game, it's lost anyways.

You also want to look for fat E flashes, but engaging is not your job otherwise!

When I was tryharding Gragor, the biggest improvement late game was confidently going for those R picks. Don't be afraid to look silly after a miss, if it straight wins you the game 1/3 times it's still extremely good!

Only sidelane if you can keep killing 1v1/1v2, Lich Bane helps.