dnd 5e – If a character has a dark vision, can he see through an area of ​​non-magical darkness filled with gas that darkens slightly?

Nothing in the rules says that having several things that make an area is considered slightly hidden makes it considered very hidden.

In a slightly darkened area, such as dim light, fog with patches or moderate foliage, creatures have disadvantages in the Wisdom (Perception) controls that depend on sight.

So, for example, if it had foliage in one place and also a dim light that did not make that space very hidden according to the rules, because very obscured has a specific definition of its own.

A very dark area, such as darkness, opaque fog or dense foliage, blocks vision completely.

In your specific example, the character with dark vision will see in a low light area as if it were slightly hidden.

There simply is not a mechanics in the rules that says how this would happen and, if so, how it would work.

Having the fog that also makes it slightly hidden does not make it be considered too hidden for that character. For that to happen, you should have something that makes the character can not see that area at all.

The rules of DM apply here naturally

Now, the rules do not specifically say that it is not allowed to stack different sources of concealment and, therefore, it is well within the power of a DM to say that having sufficient sources of concealment can in effect result in an effect that counts as very obscured . In fact, the determination of concealment is largely placed on your hands anyway, since it is very loosely defined in the rules.

Therefore, this could be a reasonable decision, but it will vary from one table to another, what the DM wants to be considered "sufficiently hidden" to increase the effect until it is very hidden.