Thank you for contacting Intel.
Intel® PRO/1000 GT Desktop Adapter's final driver release is version 18.4 for Windows* 2008.
Driver and software updates are not planned for this adapter model that's why you can't find new driver release. For operating systems later than Windows* 2008, it uses the drivers available in Windows*. Here's a list of Intel® Ethernet Adapter Supported in Microsoft Windows 8.1* for your reference.
Hope I was able to address your concerns. Feel free to contact us if you have further questions.