- is it free to play or pay to win ?
- can someone solo or does it require group play ?
Welcome to the forum. 
You have to buy the game, but that’s it. No pay to win stuff in it.
It’s mainly a single player game, focused and balanced around that, but it does have multiplayer capability if you want to play with others.
So on multiplayer, there is a trading system ? I like to farm stuff and trade.
Yes, you can trade with others. Not a system as such, but it’s possible to do in multiplayer.
There are a couple of threads here on the forum if you want to try and arrange to trade with someone.
It’s a completely stand-alone single-player game. How can it be “pay to win” when you play solo offline?
Although you can play it with your friends too, Grim Dawn is intended to be a single-player game. The current version of the game isnt stingy with loot at all (unlike early versions), so you dont have to grind 24/7 to get decent items and beat the game without overwhelming it with your corpses (like it was Diablo2).
To be honest, Grim Dawn is the best ARPG i’ve ever played, and i’m a fan of the genre. It has great depth and diversity, so you can play it over and over again. And it doesnt burden your PC either.