Preheat a stove-top grill or a BBQ. Rub a bit of olive oil on the corn and season it with salt and pepper. Place it on the grill once it's hot. You should hear a sizzling sound -- if you don't, the grill isn't ready. Wait for the sizzle or your corn won't char! Only grill the corn long enough to mark it on all sides, a total of about 3 minutes.
Cut the avocado in half and remove the skin and pit. Then, in a small mixing bowl, use a fork to mash it to the consistency you prefer (I like it a bit chunky). Add the onion, cilantro and lime juice. Mix to blend and then season to taste with the garlic salt, cayenne, and a bit of sea salt if necessary.