At a time when the base game was 16GB and most "repacks" were compressed down to 8GB, a 461GB file was an anomaly. The uploader, a user named "Vertex_Zero,"

(including the base game and DLCs) is approximately on Steam.

: You're now ready to dive into the world of Liberty City.

It sounds like you’ve come across a rather than an actual game mod.

Highly compressed or "ripped" games from unofficial sources are common vectors for viruses and malware.