which interface to use TBI, GMII or MII? (FPGA)
Well, what Ethernet PHY are you using? Pick an Ethernet PHY, then implement the interface that the PHY wants. Most likely this will be MII or GMII, depending on the speed. 

Edit: according to the schematic for your board (https://www.digikey.com/eewiki/download/attachments/64422068/DIGIKEY%20MAKER%20KIT%20REVA1_0_20170606.pdf?version=1&amp;modificationDate=1514924814090&amp;api=v2), the PHY chip is set up in GMII mode.