Most professional inventory programs are overkill for most community theatres, and the quality ones are expensive.

I would suggest using an Excel spread sheet.  Give each item a specific number for tracking purposes.  These can be as basic as L001 for a lighting instrumen and M001 for a microphone.  You can advance your part number a little by adding some descriptive numbering.  For instance, LFR001 for a Fresnel, etc. 

Once they are marked, then just enter them into a spread sheet.  Your columns could read; Part Number, Description, Storage Location, Purchase date, Original Cost, Last maintenance date, Last maintenace description, Last Physical Inventory Date (if you are going to use any type of software, you should schedule a complete inventory of your equipment at least once a year).  In addition if you really want to get ambitious, you can set up a separate sheet for each item instead of a straight listing

This would be easy to maintain, and it could be transported from member to member of the original builder of the spread sheet leaves the group.

By the way, I am an Inventory Manager for a company, so I do have experience in tracking inventory.  This is how I would handle it.

John

Microsoft Access has a built in inventory program that you can use, it also has a catagory for when bought, cost and from whom.

With gels I have found with comeatres there is not a large stock holding of rolls of gel, rather just cut from full or half sheets purchased as required.
I have seen all sorts of storage systems, from cardboard boxes to rubbish bins & less sophisticated piles in between.
A system I used for cut gels, was using open top wooden boxes, similar to a draw. With colour number# index card tab dividers.
The ends of the box are the same size as the lamp colour frame, in most cases generaly there will only be about 3 or 4 different sizes needed covering Profile, Fresnel, Flood & Par.
The length of the box would depend upon the amount of dfferent colour gels & the number of lamps used in the rig.
I use a cutting template one for each colour frame size, It is just a piece of 5 ply or mdf, with handle grip in the centre. When cutting gel I place it on a piece of masonite, as a cutting board. Then holding the frame template down on top of the sheet of gel, Use a box cutter along the edge of the ply/mdf board.
With larger rigs & where major colour change outs  happen between shows & seasons, a good idea is to have a number of cardboard or plastic conduit tubes, which are easily stacked & number indexed on a shelf. When the rig colour change happens, all the old colour is rolled, with a rubber band placed round it. Then each sepeate colour roll is popped into it's apropriate colour tube, ready for the next change back to that colour.
If you have the space, a mobile cabinet/cutting table, with shelf storage underneath, is ideal, also it can house, besides the gel boxes.  Gobo, iris, thwakers. clamps, frames,top hats, doughnuts, chains, swatchs, spares, bubbles. tools & fittings.
So everthing is one place & can be rolled out easily.
Mark all the cut gels with colour numbers, using a white chinagraph pencil, in the centre of the gel. Which helps putting it away into the correct colour section of the gel box. Also it makes it easy to identify, useing a maglite from the syage, when the lamps are in the grid.

 This may help them with colours & it might be worth it to get some sample swatch books from the suppliers.