Grammar Suggestion

There is no problem with "gently landing" itself. As you rightly say, "gently" is an adverb qualifying "landing". The issue is whether, in the sentence you wrote, the verb "landing" fits the noun "glow". It's not wrong, but to me it sounded slightly odd.

2. Glow is singular, so we use "it's" to balance subject-verb agreement, is it correct?

Almost. It should be "its", not "it's". (It's actually noun-determiner agreement rather than subject-verb agreement.)

Today, in these pleasant evening rays, different colors stepping [ or streaming would be good?] inn through my brown [adding different adjective] window, [situated word is ok?] over the tall blue, right-hand side wall, [same] I feel much relaxed -- light as fur, the dark cloud of stress and fatigue seemingly fading away.

"tall blue, right-hand side wall" is, to me, slightly too much. "right-hand side wall" isn't really grammatical, and again I question whether it helps the sentence to mention that it's "right-hand". If you really do want to include this kind of detail then I wonder if you would do better with two sentences -- perhaps one sentence to explain the layout of the room, window and wall, and another to describe the sun's rays and the feelings that they evoked.

As I mentioned before, "stepping" is the wrong word; "streaming" is fine. "Inn" should be "in". I don't understand what you mean by "situated word is ok?".

I think I answered that one before. I said: "But if you were starting a new sentence then 'seem to be' would be correct: I feel much relaxed -- light as fur. The dark clouds of stress and fatigue seem to be fading away."

My tall Off-white in color, right wall gracefully hold the blue , wide window [seems complete ?].  Today, In these pleasent evening colors streaming in through this open window, I feel much relaxed --light as fur, the dark clouds of stress and fatigue seemingly fading away.

There are a number of problems here. "off-white in color" can't go before a noun. You can say "my wall is off-white in color" or "my off-white wall...", but not "my off-white in color wall". Similarly with "right". You can say "my wall is on/at/to the right", but not "my right wall".

To agree with "wall", "hold" should be "holds", but "hold" is the wrong verb. It is not totally impossible that a wall could "hold" a window, but it doesn't sound good here.

"My wall" is certainly not wrong, but when introducing a scene I would be inclined to say "a wall". It's hard to explain why. The best I can do is to say that "my" sounds a bit "childish". That isn't quite what I mean, but it's the closest I can get. On the other hand, "my chair" is fine. The difference is that a chair is something more personal and intimate to you.

Structurally, the first sentence doesn't connect with the second as well as it might.

"Off-white" should be "off-white", there is an unnecessary space after "blue", "In" should be "in", "pleasent" should be "pleasant", and there is a space missing after "--".  Everyone makes typos occasionally; it's easy enough to do. But it's a good idea to check carefully for these sorts of "obvious" mistakes.

There are innumerable ways to rewrite this to make it better. In the following attempt, I have added "In the evenings, I... " in order to tie in the description of the window and the wall with what follows.

In the evenings, I often sit in my study, next to a wide blue window that looks out over a tall white wall. Today, as the evening colors stream in through the open window, I feel much relaxed -- light as fur, the dark clouds of stress and fatigue seemingly fading away.

You are most welcome.

Rays can be "gently landing", and, I guess, colors can be too ... that sounds OK to me.

"Gently landing" (as opposed to just "landing") is fine. It's good to include some adverbs (like "gently") and some adjectives (like "blue", or "tall") in your sentences. But don't make the mistake of thinking that you're automatically improving your sentences by stuffing them with as many adverbs and adjectives as you can think of. In excess they become obtrusive and annoying. It's all about striking a balance.

Looks pretty much OK to me! Just a couple of comments:

Today, in such a warm evening glow, different colors streaming in through the open window over [?] the blue wall, I feel much relaxed -- light as fur ...

I took out the comma after "window". The comma isn't wrong but to me the sentence reads better without it (others may have a different view). I also put a space before the dash. Spacing around dashes is optional (in proper typeset work some people use a "thin space"), but in my view if you do use them then you should use them both before and after.

To indicate that the sentence continues beyond what you've written, it's usual to use three dots (...) rather than two. (I assume that's what you meant by the two dots.)

Final point. Previously I've been assuming that "over the blue wall" meant that the colors were streaming in over the wall. This is why I suggested "different colors streaming in over the blue wall through the open window" -- because, on their journey from (presumably) the sun, the colors pass over the wall before they come through the window. It's just occurred to me, however, that you might be saying that the window is over the wall. If so, then I'd be tempted to change "over" to "above" to avoid the ambiguity.

Neither "over" nor "above" really conveys the idea that the wall actually contains the window. If that's what you want then you could say:

... streaming in through the open window set into the blue wall ...

If you want to also convey the idea that the window is high up in the wall, so that most of the wall is below it, then you could say:

... streaming in through the open window set high in the blue wall ...

But this suggests "notably high" -- probably higher than the window on the upper floor of an ordinary house.