do i need to download the game engine itself or just the run-time packages?
You need the RTP when the game creator has opted to use that feature. Their idea is that you save storage space by not having those same files for each RPGM game you have, but rather load them from one common place - which is the RTP.
If are thinking you didn't need this for some other game then it just means that the game's developer decided to include the files as stand-alone rather than use the common-location system.
This all being said, I have no idea if this is your issue. But RPGM issues are usually RTP related.
