You need an actual phisical add-on card for your computer. Those are not free but can be pretty cheap if all you want is to run garry's mod.
Depending on your computer you may need a special (older) socket cards: pci, agp or pci-e
Download cpu-z (free) here: http://www.cpuid.com/softwares/cpu-z.html
and under the mainboard tab give us the model number that is written there