tl;dr You can get Gigabit networking working on any current Raspberry Pi (A+, B+, Pi 2 model B, Pi 3 model B), and you can increase the throughput to at least 300+ Mbps (up from the standard 100 Mbps connection via built-in Ethernet).
Note about model 3 B+: The Raspberry Pi model 3 B+ includes a Gigabit wired LAN adapter onboard—though it's still hampered by the USB 2.0 bus speed (so in real world use you get ~224 Mbps instead of ~950 Mbps). So if you have a 3 B+, there's no need to buy an external USB Gigabit adapter if you want to max out the wired networking speed!
I received a shipment of some Raspberry Pi 2 model B computers for a project I'm working on (more on that to come!), and as part of my project, I've been performing a ton of benchmarks on every aspect of the 2, B+, and A+ Pis I have on hand—CPU, disk (microSD), external SSD, external HDD, memory, and networking.