No, it isn't legal, but too many users use them. Some scripts like VCMod can ban you for all server that use it, so I recommend you to don't use them. Only if you have any money.
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TheCuM Wrote: Is it actually legal to leak gmodstore addons? Or can i get sued?
No, it's not "legal" to use leaked gmod addons, but if they don't have any proofs or evidence that you don't own it, then you can probably use it without problems.