At the moment GPU's are virtually none not existent for a ok price, id suggest getting some better ram and a old i3-8th gen and then getting a GPU when the price are somewhat affordable again
@Auris PERP doesn’t require CSS anymore so this is genuinely the best option for the time being until GPU prices go down and saving your money towards a decent PC
As Garry's Mod uses the Source Engine, it is also heavily reliant on CPU power.
If you're going to spend so little, you're better off looking for CPUs of the same family generation to prevent a motherboard change. I would look around eBay for old Sandy Bridge CPUs on the LGA1155 socket. Something like this might work but I'm not too familiar with older CPU generations so I cannot guarantee compatibility.