It's not that critical as long as you stay between 1/2-to-1 tsp, whatever that weighs. Use a teaspoon measurer that's designed for measuring cooking ingredients (not an actual "teaspoon"). Such a teaspoon of salt will weigh 5g, but gelatin is lighter and so you will probably be in the 3-4g range.
As for bottling vs. kegging, it doesn't matter, since in either case the gelatin will take the haze particles with it down to the bottom of the carboy where they will stay.
Here is what I do:
1) Stir 1 tsp of Knox brand unflavored gelatin in 2/3 cup of distilled water. Most grocery stores will carry Knox gelatin. The gelatin sold at homebrew shops is no better and probably identical but a lot more expensive. Knox works perfectly (but it must be unflavored).
2) Microwave the gelatin mixture in 15-second increments to get up to 160F, stirring between increments. Be careful not to exceed 170F or you will have Jello instead. At 160F the gelatin will be sufficiently sanitized for addition to your beer.
3) Pour the gelatin mixture into your carboy.
4) Keep the carboy in the fridge until the beer is clear (approx. 2-3 days). The gelatin will not clear well if the carboy has not been kept cool.