According to d20pfsrd, the major difference between a major and minor artifact is how many there are. It even point out that "Minor artifacts are not necessarily unique items", just that they merely can no longer be created by common mortals. A major artifact on the other hand is one-of-a-kind, there are no copies and most likely never will be.

The problem with balancing or measuring the power of artifacts is that they are intended to possess unimaginable amounts of power. I would not worry about trying to balance the artifact with others 1) because they are not intended to be balanced and 2) because you'd have a difficult time creating something vastly more powerful than those already created. The main point of artifacts is to drive the plot, usually by either destroying or protecting them.

Finally on the point of actually creating a custom artifact. This has already been covered by a question here: Do rules exist for creating artefacts? and general consensus is that there are no rules to create an artifact.

In short, artifacts are incredibly powerful and not meant to have any balance with other items. They should mainly be used to drive the plot. And finally, there are no rules about creating them, so go wild with it; do whatever will make your campaign just a bit more epic.

Following Wikipedia's Garl Glittergold publication history, I perused Deities and Demigods (1980) and the Dragon #61 (1982) article "The Gnomish Point of View" (28-30) for AD&D, Monster Mythology (1992) and Demihuman Deities (1998) for AD&D2E, Deities and Demigods (2002) and Faiths and Pantheons (2002) for D&D3E, and Complete Divine (2004) for D&D3.5. This is in addition to a handful of other sources. You needn't've worried about being overwhelmed; there ain't much.

Garl Glittergold's battleaxe Arumdina

The Encyclopedia Magica (1999) for AD&D2E lists Garl Glittergold's traditional weapon in Axes under Arumdina (112) and Garl Glittergold's Battle (118). In both entries Arumdina is given statistics to indicate that it's "a unique and potent item (often called an artifact or relic)" and that it "is so priceless that it is impossible to rate its true barter value."

Although Deities and Demigods for Dungeons and Dragons, Third Edition downgrades Arumdina to a nonitelligent (yet still epic) +5 dancing ghost touch speed battleaxe, Arumdina is said by multiple sources to be intelligent as well as capable of cleaving through stone as easily as air and, in most cases, once per day of fully healing the god of any damage that's been dealt him.

Further inspiration

Here're some things to consider if you want to homebrew your own.

• Monster Mythology says that Garl Glittergold embarrassed Grankhul, hunter god of the bugbears, by masquerading as a deer. A quest for Garl Glittergold's sacred deerskin cloak or something isn't bad.
• Faiths and Patheons says that Garl Glittergold has "the largest collection of jokes in the multiverse" (135). A copy of his jokebook would certainly be an entertaining basis for an artifact.
• Complete Divine's gem of the glitterdepth (97) increases a devoted Garl Glittergold worshiper's Wisdom and gives him a sense of his surroundings while spelunking, alerting him to the location of floors, walls, and ceilings.
• Complete Divine mentions offhandedly as a quest for Garl Glittergold worshipers "seeking the long-lost Ebondark Gem Mine on the Elemental Plane of Earth" (112). A map to it or the gems themselves could be artifacts.
• Magic Item Compendium's hooked hammer of the hearthfire (52-3) is a combination hammer and pick that, when wielded by a devoted worshiper of Garl Glittergold, bursts into flames that damage those that are struck by it.