Anybody can gain muscle mass. The only differences are where they start from, what they start with and what might be making it more difficult for them.
We all start with our own foundations. We all fall into different body types. Although we are not trapped by these types in the way we cannot do anything to change our body, we will always be working with what we were given. And so if you gain fat easier, you will always be able to do this. If you gain both muscle and fat easier, likewise you will always be able to do this. If you struggle to gain muscle then again you will always start from this position.
That means you will lose it faster and have to work harder sometimes to gain muscle that perhaps the big dude in the gym doesn't have to fight as much for like you do. That's just life man, you can't beat yourself up over it!
The good news is YES you can put on size, muscle and strength. You can do all that. The only determining factor is how dedicated you are to making that happen and what you are willing to do to make it happen. If you wanted to become a bodybuilder - you could do it. Would you be able to put on the same size a Jay Cutler or Ronnie Coleman could? Debatable. Would you be able to have the chest and arms of an Arnie? Probably not. But that's because you are none of those dudes. Its not because your body is any different to them in the way it can increase in size, strength and mass. But if you really wanted to you could blow up to something that wasn't world's away to how they were in their prime. You wouldn't BE them but you could get your body to CHANGE like theirs did and reach a point where you are massively different to the day you started hitting the weights.
And that's the most important part to remember. You can do it. Its all about the journey. It sounds impossible if you started off with a skinny frame and you look around you and everyone naturally looks bigger than you, but it's not