Race 07 is the base game. Nothing will work if you don't have that installed. Race On and GTR Evolution WILL NOT WORK if you
don't have Race 07 installed.
Race On and GTR Evolution are honestly, the real expansion packs.
Race On adds 7 classes. 4 are production, and 3 cover other things. Race On also adds famous American tracks, and the 2008 WTCC Tracks. One of which you can't find in any other pack or GTR Evolution.
Now, if you decide to run Race On instead of Race 07 on Steam,
You will get a different user interface. The same goes for GTR Evolution. So, If you like a certain interface, run the game
from the game of your choice.
While running the game from Race On or GTR Evolution gives you a different user interface, screenshots and playtime still go to Race 07.
GTR Evolution is NOT the same as the GT Power Pack. As GTR Evolution adds 7 car classes 4 of which like Race On, are production. GTR Evolution adds 3 tracks, all which are
the famous Nurburing tracks.
STCC2 adds newer (and some brand new) cars than STCC. Also,
the tracks in STCC2 are the 2010 STCC Tracks with a new one.
STCC adds older STCC cars than STCC2. Also, the tracks in STCC
are the older STCC tracks. There is a track in STCC not found in STCC2.
Also, STCC adds real life driver pictures for certain liveries instead of flags. Which if you ALSO have STCC2, certain
liveries found in STCC2 will have the real life driver pictures.
WTCC10 adds four newer WTCC cars (and some new). And the 2010 WTCC tracks. With a new track added.
GT Power Pack only adds 3 tracks and 3 cars. However, you cannot
find any variations of the cars and tracks found here in any other pack or expansions.
The Retro Expansion is rather poorly named as it's NOT a real
expansion in the normal sense. It's just DLC that adds 2 retro
car classes and a hillclimb track. You cannot find any variation
of the stuff you find here in any other pack or expansion.
Race Injection is the biggest offender as far as misleading product details go. It's just Race 07 with every pack BUT
STCC, Race On, and GTR Evolution. There is no bug fixes, no
new content, no well nothing. If they were to make a "last hurrah", you'd think they would add everything!
In a nutshell,
Race On is an Expansion Pack, Race 07 is the actual game.
Running Race 07 from steam changes the user interface. If you run the game from
Race 07, if you installed Race On, you get Race On's content, but you get Race 07's
user interface.
GTR Evolution adds completely different things than GT Power Pack.
GTR Evoultion also adds a new interface if you choose to run it in steam instead of Race 07
STCC2 adds a couple new cars and newer version of STCC's cars.
STCC and STCC2 both have the same tracks but both have one track exclusive to each pack.
Retro Expansion is not a real expansion, it's DLC.
Race Injection does not add anything new, and anything you find in Race Injection
you can get in Race 07, but you'll have to pay a bit more. Since you already bought
Race On and Race 07, it's cheaper to just buy the rest of the DLC instead of Race Injection.