From here, I found such a sentence:
This Gem won't cover hardware details...
Besides, I also found a book named:
I could not look up the meaning of "Gem".
What's its meaning in this context?
I have investigated and Robin seems to be right. The Gem is referring to a book called "Game Programming Gems" and in this case refers to a small but valuable piece of information of which there are more than one of in the book.
From the author of the article you quoted:
I posted two articles I wrote for Game Programming Gems 8[amazon.com]. I was able to keep the copyright for publishing on the web, so I can post them here. The lamer of the articles is titled "Secure Channel Communication" (138K PDF) and is an overview of items to consider when designing game network protocols. The better article is "Introduction to x64 Assembly" (368K PDF) which covers many aspects of assembly programming on x64 systems, including register set overview (wuth MMX and SSE), assembler support, interfacing with C/C++, and details on the calling convention used on x64 systems. Both papers are on my papers page.
Intel replaced Gem with White paper here:
Gem is often used to mean "a small item of great value". As such it is often used to describe small pieces of factual information that are very useful. Generally it would be unseemly to refer to ones own work as such (I might call an answer on this site a gem, but to do so of one of my own would be boasting), but marketing requires one to make boastful claims of ones work.
This sense of gem could be used of a collection of such small items, as in Game Programming Gems, a series of books by someone from Intel, and so on.
Within the context of doing so, it would make sense to refer to each essay as a gem, even within the essay.
As such, "This Gem won't cover hardware details" would mean that particular essay won't cover hardware details. Out of context, it no longer makes sense.
So it's not jargon, it's marketing.
It is unclear what the intent of the author was. If the word is meant to be an acronym then it should be all caps as in 'GEM'. If the word is meant to refer to a brand name or object then 'Gem' is correct but as the word is not used elsewhere in the document, nor is any external reference made, and neither is 'Gem' a common computing term, we have to conclude that the author has mistyped the capital letter and meant to write 'gem'.
A gem is a small precious stone, it appears to be used here to imply that the following information is concise and valuable. A 'gem of wisdom'.