What are the differences between these motherboards?

Solution
From what I can see, the Asus has 2 more internal USB 3 ports than the Gig.

The Gig has 2 rear 10GB/sec USB ports, while the Asus has 2 rear 5GB/sec USB ports.

PCIe slots: Gig x16, x1,x1 (3 total) Asus x16, x4, x1,x1 (4 total)

Maximum Memory: Gig 2 x 16, Asus 4 x 16

SATA Connections: Gig 4, Asus 6

M.2: Gig 0, Asus 1

The UEFI on the Asus is probably nicer.

Asus Pro's: more PCIe slots, better UEFI, up to 64 GB DRAM, more SATA connections, M.2

Gigabyte Pro's: 2 x USB 10GB/sec ports

I doubt you will need those extra internal USB ports. I would go with the Asus.
From what I can see, the Asus has 2 more internal USB 3 ports than the Gig.

The Gig has 2 rear 10GB/sec USB ports, while the Asus has 2 rear 5GB/sec USB ports.

PCIe slots: Gig x16, x1,x1 (3 total) Asus x16, x4, x1,x1 (4 total)

Maximum Memory: Gig 2 x 16, Asus 4 x 16

SATA Connections: Gig 4, Asus 6

M.2: Gig 0, Asus 1

The UEFI on the Asus is probably nicer.

Asus Pro's: more PCIe slots, better UEFI, up to 64 GB DRAM, more SATA connections, M.2

Gigabyte Pro's: 2 x USB 10GB/sec ports

I doubt you will need those extra internal USB ports. I would go with the Asus.
 
Solution