A player of "Metal Gear Solid 3: Subsistence" unlocks the "EZ Gun," a silenced
tranquilizer weapon with infinite ammunition, if they capture every type of
edible plant and animal. This U.K. Yahoo! Games "Cheats and Tips" page points
out the way.
"EZ Gun ... Capture all 48 types of edible plants and animals"
The above page also indicates the other way to unlock the EZ Gun.
"Playing the game on Very Easy mode."
This Gamespot game guide provides more tips on this unlock.
"You must capture all types of animals and plants in the game, you dont have to
eat them and they can be rotten, and once you have captured one you can discard
it from your backpack because it has been recorded as it having been caputred,
there are 48 in total to be captured."
If you need any clarification, please feel free to ask.
Google search on: "Metal Gear Solid 3: Subsistence" food
Looking Forward, denco-ga - Google Answers Researcher