The poems were all created just for the film. From The National Board of Review:
Anderson (The Director): This is very rude in fact, but Ralph was always very good with text, and he has to recite all this poetry. But
the poems in the script get chopped off where I stopped writing them,
and Ralph always continued on and gave me a couple extra stanzas. In
the earlier cuts of the film they went on, and you were very good at
Interviewer: Is it correct that you wrote the piece for Ralph?
Anderson: I had wanted to do something with Ralph for years and years and then we met and got to know each other over the past three
to four years. Ralph had shown me a sort of mood reel for the film
Coriolanus, and he did some of the speech, and in that moment, I knew
I had to get Ralph in a movie. Just seeing him do Shakespeare on a
sofa. We didn’t really think of any of the casting until the script
was done, except for Mr. Gustav. We had Ralph in mind, and were hoping
he’d be seduced into doing it.
He refers to the apple as a son of a bitch initially simply because he's a quirky, passionate character. It's a loose phrase and is more a revelation of his frustration that people will fight him for it than a comment on the painting itself.
The poem he breaks into certainly is describing the painting. A bard is effectively a professional lyricist/poet. M Gustave's lines are saying the poem is so beautiful, it's beyond even the description of them. All they can discuss is the lack of her and all she isn't, i.e. things that the painting is missing - they can't sing about just how beautiful it is because its beauty defies description.
And it's certainly indicative of M Gustave's weird character. Throughout the film, he descends into little lines of poetry, often leaving them incomplete due to interruption. In fact, it's not even clear how much meaning they have to him, or if they're simply being recited from a nostalgic viewpoint.