By gain - you mean you increase your HP maximum or heal for the amount?

I'll use as healing...

Whenever I make items like this my party wants to attack every squirrel or simple animal they see to try and regain health, or they even try to justify attacking trees. What's the logical argument to not allow this? Add a required enemy CR to the item?

Good question, zwand. My players are different kind of players, so I won't see this problem appear quickly. However, I'd rule the following.

Attacking a tree is very easy to do; you can't miss, so you don't have to roll an attack roll.
You can do the same with a squirrel. If the squirrel is no match to their might, just make them roll for damage (or make it into a skill challenge instead of a fight). The damage roll adds nothing to the narrative. 

Hello Zwand19, I know that I am late to the party, but this issue is extremely simple. You cannot perform a critical hit on any inanimate object or non-hostile creature who is running away or not fighting back. Crits are the direct result of competitive marital combat and are thus the product of combat. rolls. Tell players to keep in mind that "hits" in general do not refer to physical injury, and hit point losses do not mean there is a physical wound involved.  Hit points are the expression of one acquired skills and abilities as much as a measure of one's physical endurance.  So as abstract as that is, so too is the concept of a critical hit.  :-)