See above.1) The sun was hiding behind the building and our garden was seized by the shadow bit by bit until it got completely dark.
"Hiding" is fine but I don't think "seized" works.
2) The sun was hiding behind the building as if trying to flee from the moon that had just appeared on the east horizon.
(My own piece of writing)
I like this - I would probably say "the moon that had just risen in the east" though, or "just appeared over the eastern horizon".
Would you understand 'the sun was hiding' in 1) as a gradual process and 'the sun was hiding' in 2) as if the sun was already there, in other words, the sun was hidden behind the building (my apologies, but I seem to be unable to label it).
I don't read it as a gradual process. It just means to me that the sun was behind the building. I imagined it like a small child playing hide-and-seek and hiding behind a tree. You have given the sun human characteristics - I like that.
I'll try to paraphrase the sentences to try to be more clear about what I want to convey.
1) The sun was disappearing gradually behind the building and our garden was seized by the shadow bit by bit until it got completely dark.
For that meaning, I would use "disappearing" or perhaps "sliding behind the building".
2) The sun was hidden behind the building as if someone mighty enough had put it there trying to conceal its presence from the moon.
"Mighty enough" for what?