Gigabit NICs are only for networks that have Gigabit connections. Meaning if at home you have a computer with a Gigabit NIC and you transfer a lot of files (especially large ones) between your PC and your laptop, then having a Gigabit NIC in your laptop would make sense.
Internet connections do not operate at 1000Mbps. They only operate at upto 100Mbps so getting a Gigabit NIC will not increase your internet bandwidth for faster downloading/uploading.