Steam allows modding (i have Steam myself) . then according to your liking just browse the DL sites for what you'd like . Gostown is a standalone map , if you install it you won't be able to play GTA IV unless of course you made a backup which i highly suggest you to do . i won't enumerate all faboulous mods out there as it would take days to complete . but here is the "basis"
- xliveless asi loader : allows asi scripts loading and to bypass GFWL login . be sure to dl version for your game version
main tools :
- open iv : poweful modding tool . allows textures import/export , objects localization on map , wpl editing , audio viewer and much more
- GTA vehicles mod installer : allows vehicles install in seconds , and reverse to original vehicle in seconds too
- edit iv : for timecyc editing tutorial
- GTA IV handling editor : for handling editing tutorial
mods i love a lot :indicators script