You can't even
"nail" the sound using the exact same equipment and settings used by Jerry (? Bob? Mickey? -the way the question was phrased its not even clear we are talking about guitar... Eileen Law? Laird Grant?

) - why? because what Kevo said is true- it's mostly in the fingers, or more properly, in the player as a whole. Nobody has ever truly nailed Garcia... Kadlecik, Mattson, myself, Barry Sless, Stu Ross... NOBODY.
However, YES there are probably hundreds of choices for getting a single pedal w/multiple effects that will
"approximate" many of the types of effects used by Garcia or Weir... sure, find one with an envelope effect and most will include an overdrive, a delay/reverb, and maybe a phase, and you'll be able to combine them all. But as Mike above pointed out, you will probably soon start to notice the shortcomings and/or cheesiness and/or inflexibility of these type of "does-it-all" kind of boxes.