YEs you need a 3rd model software to make models.
Gtos models where made with studio 3dmax 2008. Max is stupid expensive. So you dont want to go that way.
Blender is free. I have messed around in it but am so use to max it just ticked me off. to learn to make models, unwrap and texture them takes time. And can be quite alot of time. If you take to it, its like crack. I would try blender. It can make nice models. But you will be investing a ton of time to learn to do it well. Game mod assets are made out there a plenty in blender. Some real nice stuff has been made. BUT....... most folks who start out totally do not understand the basic fundamentals of models for games. IE.... they make something like a gun with 30000 triangles and no LODS. Which is totally insane. Or go make a tank with 500000 triangle and expect that to work well in a real time rendered environment. It does not....
Photoshop is the most useful program EVER to make textures. It costs money. Most use the thing to just mess around in photos. That is like 1/20 of what it can actually do. Freaking great program that keeps getting better. GIMP is a free program that I also sometimes use for certain things. It is good enough to make do quite alot of stuff.
Here is what I would suggest... Get blender and gimp. You will be watching alot of internet videos to get how the hell to do anything in it. Then pick a vary simple item to make. Like make piece of soldier gear. Even a simple low poly gun. Learn to model. Look at similar model in game and look at how many triangles where used to make it. You want to stay close to that. A GTOS gun is between 500 and 1000 triangles. So DON'T make a 30000 triangle lugar. You then need to learn how to unwrap it. This is setting up how a texture is layed out. It can be a bit painful and is my least liked thing about making models. Its freaking annoying. Then you make the texture that is applied to the model. Generally the vehicles use a 2048 X 2048 pixel texture for tanks. Guns I think are 512 x 512. This is low poly modeling for games. You want to use the smallest amount of everything that looks good. The game has to render LOTS of this stuff. You go crazy with triangles and texture size, the game will run like crap. Does not matter what game it is.
Ok... then.... You have a nice piece of gear or a gun with a texture on it. NOW...you need to get it ready for the game. The game uses LOD (level of detail) So your piece of gear in game is actually 2 or 3 models. Depending on how far away from the camera it will use lower polygon models to render. This is VARY important for a game the size gtos. So you either make 2 or 3 models from scratch using the same texture or you make the highest poly first and then "optimize" it by removing unneeded triangles and vertexes that would could not tell where ever there if the model was a set distance from the camera. So a 1000 triangle gun a 0 meters the model is DUH...1000 triangles. At 50 meters distance you need to make the 2nd LOD be say 600 triangles. At 200 meters it might need to be 300. Etc. Not set in stone.
Well there's more. but that is the basics in 2 paragraphs. But the important part for the budding model maker for games is.... Use the least amount of everything that looks good. Where every new mod guy that makes something totally screws up is not understanding this. Why so many mods run like crap. A usual FPS sort of game the studio makes a 2000 to 4000 triangle gun now a days. But some ding bat makes some monstrosity 35000 triangle gun with no LOD's. Its generally fine if only a few are on screen (its wasteful but runs). But total HORRIBLE fps killer if like 40 are on screen. Maybe even 4.
Have to learn to get the max out of geometry and fake 3d space where ever possible with textures. And you generally get 3 types of texture to do this. Defuse (normal color texture), normal maps (they fake 3rd look on flat surfaces), and specular maps (show light and dark areas with light).
Ok I set out the basics. But dont worry about anything other than make a nice model at first. then worry about getting it ready for textures. Then texturing it. Then making the model ready for game (LOD's) and finally whatever rigging needs to happen so the game knows WTF to do with it.
