Krita > GIMP
It really depends on what kind of game you're going to make. Some games don't have AI, some games don't use 3D, some games can get by with stock sounds and music. Typically a developer has some clear strength and some clear weakness, and so they focus on games which highlight their strengths and minimize their weakness, e.g. someone who is most comfortable with art might make a game that's art-heavy but light on other aspects, such as a point-and-click adventure game. Other times a developer will be more of a jack of all trades and can make just about anything (so long as it's relatively small due to the limitations of being solo.)
Personally I don't like to think of my present skill set as a limiter of what I like to create, and anything I come up short on I will devote to learning. The obvious consequence in doing so is that it takes a lot longer to ever create something, as a lot of your time is spent in developing skills instead of making games, which can be demoralizing as you don't get to see the fruit of your labors for a much longer time than if you compromise on your vision and deal with whatever your limitations are. Some people just can't play this long game but I was blessed with patience. In my case I had to learn programming from a state of never having programmed before, and that's taken not only the longest amount of time but also has proven to be among the most important skills to acquire in relation to game development (unless you wanna wind up using RPG Maker or something, the products of which rarely get attention.)
I'll say this: if there are many skills you need to develop at once to approach a respectable level, esp. if you've never worked on some of those skills before, it may be better to work with at least one other person who can make up for your worst abilities, or to pay someone to take care of a particular aspect of the game. I've noticed this is usually done in the form of commissioning a musician to create the music and sometimes the sounds for a game, likewise typically a programmer will pair with a designer/artist combo. Being great at music, sound design, graphic design, illustration, animation, and programming is a pretty rare thing, considering the sheer amount of time involved in both honing those skills and applying them to make a product, and a game that is made exclusively by one person will usually suffer in some aspect.
I am always skeptical when people say X game was made by one guy, because there were usually more people involved in the project than this implies; you might say X game was designed and mostly created by one guy who had help to get closer to the truth for a lot of projects. I can think of Cave Story, Stardew Valley, Axiom Verge, and Undertale being pretty much one guy doing everything, whereas Fez, Braid, Dust: An Elysian Tale, and Minecraft are frequently cited as being by one guy despite them having other people helping out in some form. So I suppose you could extend the definition of "made by one guy" into "funded by one guy", since the one guy still has primary control over all aspects of the project, but I think there's an artistic distinction between the two; one is completely the product of a single person, the other is primarily influenced by a single person but may contain the creative products of someone else.
In any case if you don't mind hiring people for a project that shows promise then I'd say you need to know how to program first and foremost; you'll never get off the ground as a solo dev if you can't do that.
