GEM, or the Graphics Execution Manager, is a memory-management implementation spawned a month ago by Intel to address the shortcomings of the TTM memory manager, which has been in development and the talk of many X.Org video driver discussions for the past few months. GEM vs. TTM has turned political in mailing list discussions and elsewhere due to Intel's decision to suspend their TTM work and focus all of their resources on writing this GEM code instead of just working to improve and fix TTM.
TTM was fathered by Tungsten Graphics. The advantages for this video memory manager is dynamic tracking of buffers, full access to all addressable video memory for every 3D context, a fence facility for hardware synchronization, and a guarantee that textures and video memory buffers will be preserved. Intel views its GEM implementation as much simpler than TTM while still offering all of the memory management benefits.
Once Intel's GEM code is merged to master they will then proceed to strip away all of the TTM code currently found in the Intel driver, completely drop the existing TTM user-land API, and the libdrm version will be bumped to 2.4.0. This 2.4 version of libdrm will then be a requirement for future versions of the Intel X.Org driver.
Speaking of a new Intel 2D driver, Eric Anholt expects to release xf86-video-intel 2.4.0 next month. xf86-video-intel 2.3.0 was released back in April and this July update will be their quarterly update. This driver will contain a number of fixes and other work that we've now been talking about in a number of articles.