One of the things that Dungeons and Dragons has in profusion that GURPS lacks are consumable magical items. Certainly, we still have one-use items like scrolls and potions, and there are mana stones for mages, but in the place of wands et. al. with charges, we've substituted items of infinite use whose drawback is instead that they draw on the user's FP.
Unfortunately, this is not an equivalent solution. There's quite a bit of cleverness in limiting the use of a magic item; it allows for imbalances to naturally self-correct, and it adds an interesting dynamic of resource management to the use of the items. (I contend that managing non-renewable resources is inherently riskier than managing renewable ones, because the stakes are higher. Accordingly, the payoff can be bigger, and the act of deciding more interesting).
Here's where if I were less interested in mathematics and fun, I'd suggest waving your hands about mana-stones and just giving the items charges and be done with it.
I've always disliked the system of charges. It's too easy to measure, and it requires meticulous book-keeping. Boring, I say.
On the way home from work I had a flash of insight. GURPS magic items already have a statistic that doesn't usually matter much: Power. Power is usually 15, and is equivalent to the effective skill of the enchanter. Mostly it seems to be used for resistance rolls and determining if the item functions (e.g. low mana zones).
Wait, it's equivalent to a skill, so what about a roll-under mechanic?
Every time a PC uses a magic item with limited uses (usually things that let you cast spells or give you temporary effects, like rings of invisibility or fireball wands rather than magic swords), the player rolls 3d6. If he rolls at or under the current effective Power of the item, it behaves normally. If he rolls above the current effective Power of the item, the effect still happens, but the Power of the item is reduced by 1. (It's more stressing to a wand to use it in a low mana zone.) An 18 is always a failure.
Once the item's power goes to 0 (or 2, if you like - it's a difference of two uses), it is depleted and no longer counts as magical. Maybe it can be recharged, maybe it can't.
To get a feel for how this would work, I ran the probabilities, with my target at 75%; that is, I figured out in how many rolls it would take for the item to have a 75% chance of depletion, based on starting Power:
Power 14: 37 rolls
Power 15: 66 rolls
Power 16: 140 rolls
Power 17: 434 rolls (here's where the exponential progression breaks to a purely arithmetic one)
Power 20: 1317 rolls
The system isn't perfect; over a thousand rolls is effectively infinite. However, you can easily assign Power yourself for items the PCs find in the dungeon. Interestingly, this lines up nicely with the enchantment rules in GURPS Magic, which point out that most items will probably have Power 15. (I'm ignoring the bit where they need Power 15 in order to function, of course. I will keep this for swords and other always-on items, but for consumables I'm using it in a different way.)
What about permanent items? If you want to keep some, as truly powerful artifacts, I'd recommend just assigning them a Power of 25 or 30 and ignoring the book-keeping. At the same time you can ratchet up the price for a high-Power item as recommended in Magic, not to mention a premium for the rarity, if this thing even has a sell-price.