What are MGT pins used for in fpga and what devices can they be connected to? How can I connect them on fgpa system planner?
MGT pins are dedicated pins for high speed serializers.  They usually come in sets of four - differential TX and differential RX - along with some dedicated reference clock inputs.  These pins can be used for high speed serial protocols including USB 3, SATA, PCIe, several flavors of Ethernet, etc.  Operation at these speeds requires certain electrical characteristics, so the MGT pins are necessarily dedicated pins that are directly connected to the serializers on the FPGA.  They cannot be used as general-purpose IO.