Early 2008 iMac with El Capitan not getting Gigabit speed

+1 on Ikrupp's answer. Your expectations are not really realistic. If you want faster speeds you need to have the connection speed at the modem and the hardware in the home to support it, including the router and a new Mac that supports those speeds.

Your physical layer Ethernet cable connection will show 1Gbps, but your real-world performance would be at best, an eighth of that (125 MBps) owing to 8-bits per byte. And that is very unlikely on a 2008 iMac with thirteen-year-old chip technology and by current standards, an ancient operating system.

Not much to be done with a 13 year old computer. It’s entirely possible that the 2008 hardware/software simply can’t handle it even if the iMac has a 10/100/1000 ethernet port.

You are welcome.

Happy to assist.