All Gigabyte boards are prefixed GA-, just that some (r)etailers do away with the prefix (Gigabyte also use GV- as a prefix for their graphics cards)
Regarding the comparison:
The -
D3H is an entry level board. The 4-pin EPS12V denote that the board isn't aimed at the overclocking crowd, and functionally the board is artificially limited to one full speed PCI-E x16 with the second x16 slot capped at x4 ( certified for CrossfireX but not SLI if you were planning on running two graphics cards. No big deal if you're looking at a single card). The biggest differentiator is the bare bones USB3.0 support (2 ports rear I/O, 2 via onboard), and SATA ports (chipset supported 2 x SATA 6GB and 4 x SATA 3GB)- the crappy old school vertical SATA port placement is another signpost to Budgetland, as is the lack of DisplayPort video out connection.
The
(GA-) Z68AP-D3 can be regarded as the earlier Z68 chipset version of the Z77-D3H.
The
(GA-) Z68XP-UD4 is the mainstream board from Gigabyte for last years Z68 chipset. Note that the PCB is now a fashionable black (used with Gigabytes performance boards), the EPS12V is now 8-pin (overclocker orientated), a beefier 16+2 phase power delivery (the cheaper boards use 6+2. Also note that the voltage regulation is fully heatsinked)-also aimed at overclocking, and some extra connectivity via third party controllers -an extra 4 SATA 6GB ports including two eSATA, and the second PCI-E x16 slot is now rated at x8 (CrossfireX and SLI). If you're not overclocking and don't need the extra SATA ports and seven extra USB2.0 ports then the boards offer the same basic level of functionality.
Gigabyte Z68 boards use a
conventional BIOS, their Z77 boards use
UEFI. Some Gigabyte Z68 boards exhibit reboot loop behaviour. This seems to have been allieviated (but not entirely cured) with BIOS updates and culling the suspect boards from stock. Some boards show the tendency, some/most do not. If you opt for the Z68 board and it does go through a cold boot reboot loop look to RMA it -chances are that no troubleshooting (settings change, BIOS update etc.) will likely cure the issue.