This is a great little app - thanks for developing it.
A few ideas for improvements:
When editing and item (correcting the weight for example) the change shows on the master list, but not in the trip lists. Not sure if this was intentional but it would probably make sense if any change to the master list propagated to the trip lists.
You could name the master list "gear closet".
The ability to re-order items in the list would be great, right now they show in the order they were added.
Could you do sub categories? Some might find it useful to be able to add subcategories such as navigation, raingear, first-aid, etc to make the lists more readable.
Sometimes I bring more then one of something such as: days of food, liters of water, pairs of socks, tent stakes. Maybe add number of items and have it default to one.