It supports 10/100 Mbps (Fast Ethernet), not Gigabit.

Official support for the RTL8139 family generally ended with Windows 7/8. However, these cards can often still work on Windows 10 using the following methods: Go to product viewer dialog for this item. Realtek RTL8139D 10/100Mbps Ethernet Card - PCI

If you are looking for a replacement or a modern equivalent for a Windows 10/11 desktop:

: For the most reliable software, visit the Realtek PCI/PCIe Ethernet Controllers download page. While the RTL8139 is technically a 10/100M Fast Ethernet controller (not Gigabit), newer PCIe versions often use similar driver packages.

One of the most interesting technical aspects of this specific driver on Windows 10 is

Go to the Realtek download center.

A significant upgrade for modern systems, providing 1 Gbps (1000 Mbps) speeds over the PCI-E bus.

These chips were the "workhorses" of the early 2000s and are known for their extreme durability.