What a GI protects
A geographical indication does two things: it fixes a boundary, and it fixes a method.
For Chinese green tea the system runs through national standards rather than through a European style register.
GB/T 18650 defines Longjing（龙井）by production area and by the steps used to make it, and it allows three areas rather than one.
GB/T 18957 does the same for Dongting Biluochun（洞庭山碧螺春）, tying the name to the two islands in Taihu.
GB/T 22737 covers Xinyang Maojian（信阳毛尖）and sets out where the leaf may come from within Henan.
What these documents do not do is grade the tea. A GI tea can be poor.
Certification of origin is also a different thing from organic certification, which covers inputs rather than place.
A tea may hold one, both or neither, and a GI mark tells you nothing whatever about pesticide or fertiliser.
The main Chinese GIs
Longjing is the most borrowed name and the most protected, with a mark administered locally and a standard that covers Xihu（西湖）, Qiantang（钱塘）and Yuezhou（越州）.
Biluochun is smaller and easier to police because the growing area is genuinely small.
Xinyang Maojian is protected at the prefecture level, which leaves a lot of room inside the boundary.
Anhui protects Huangshan Maofeng（黄山毛峰）, Taiping Houkui（太平猴魁）and Liu’an Guapian（六安瓜片）separately, which is unusual and useful.
Outside China, Darjeeling is a registered GI and its green tea carries the mark, though most Darjeeling is made as black.
Japanese tea has no equivalent national GI for most green tea, and the system there runs on prefectural brands and competitions instead.
Most of the Chinese GI standards for green tea were first published between 2002 and 2008, and several have been revised since.
A revision usually widens an area or clarifies a method, so the version cited on a listing is worth a glance.
None of this is unique to tea. Cheese, wine and olive oil all run on the same principle, and all of them have the same gap between the protected name and the quality inside the packet.
Knowing the gap exists is most of what a buyer needs.
What the label leaves out
Three things a GI never tells you: the grade, the harvest date, and whether the leaf came from the core or the edge of the area.
Grade is set separately, and for Longjing the standard describes grades by leaf appearance rather than by taste.
Harvest date is the single most useful fact about a green tea and the one most often missing.
Pre-Qingming（清明）leaf from the edge of a GI area is usually a better buy than a late picking from the middle of it.
And a GI mark says nothing about storage. A certified tea that spent a summer in a warm warehouse is worse than an uncertified one that did not.
Certification protects the name. It does not protect the leaf in the bag.
Two batches from the same area at the same grade can sit a factor of three apart in price, and the picking date is nearly always why.
Price spreads like that are a signal that the label is doing less work than the seller implies.
Checking one yourself
Start with the standard number. A seller quoting GB/T 18650 has at least read the bag.
Then ask for the area, not the province. Xihu, Qiantang and Yuezhou are different answers with very different prices.
Then ask for the picking date, and be wary of any green tea sold in late summer as new without one.
For Longjing specifically, check whether the mark is the local Xihu one or the general standard one, because they are not interchangeable.
If the seller cannot answer any of the three, the GI mark is decoration.
A certification is a floor, not a ceiling, and the useful information is always the specific one nobody printed.
When all three answers come back quickly and specifically, the tea is usually what the bag claims.
When they come back as a lecture about terroir, assume the cheapest leaf the standard allows.
Keep in mind what these marks were built for, which is protecting growers from outside leaf, not protecting buyers from bad tea.
Used that way they are genuinely useful, and used as a quality guarantee they let people down.