https://www.gigabyte.com/Comparison/Result/2?pids=6071,6072,6073
Short summary:
-- Gaming 7 & 8 have a Thunderbolt 3 connector for onboard graphics, in addition to the DisplayPort & HDMI ports, allowing them to support up to 3 monitors on the iGPU
-- Gaming 7 & 8 have a Creative Sound Core 3D audio chip, with different amplifiers (the 7 & 8 share a TI amplifier, but the 8 adds two JRC amplifiers)
-- Gaming 5 supports 2.1, 4.1, 5.1 & 7.1 surround sound; 7 & 8 only support 2.1 & 5.1
-- Gaming 8 has a 2nd PCIe x4 slot. Unlike the 5 & 7, neither x4 slot drops down to x2 if an SSD is connected to the M2P_32G connector; however, if you install an item in x4 slot #1, then your x8 slot drops down to x4 speed.
-- Gaming 8 adds an additional U2 connector, & a separate controller for 2 additional SATA III slots.
-- Gaming 7 & 8 trade out the 2 back-panel USB 2.0/1.1 ports for a 5th USB 3.1 Gen1 port
-- Gaming 7 adds 2 additional system fan headers, a 2nd system fan/water cooling pump header, 2 temperature sensor heads, Clear CMOS, power & reset buttons on the motherboard, 2 BIOS switches, & Voltage Measurement Points; Gaming 8 adds an "I/O shield audio LED power connector" (whatever that is) in addition to the Gaming 7's options
-- Gaming 8 has 2 MMCX antennae connectors on the back panel
To be honest, unless you're planning on supporting triple monitors with the iGPU, or are planning to switch all of your peripherals over from USB2.0 to USB3.1 (Gen1 or Gen2), I don't see much of a point to either the Gaming 7 or 8, so I'd go for the Gaming 5.