Even though I'm only up to Ep.3 I'm loving the anime(though the manga is better) and I've always noticed its music. The music feels really familiar even though I'm pretty sure most of it is new(I wouldn't be surprised if they used some well known compositinos though) and reminds me of other animes. For example, in ep2 right after Keroro escapes from Natsumi's gym class the music changes and it sounds like an alternative take on "Colorless Sky" from Trigun. I wish they would release an OST.
Keroro Gunsou has 3 OST's currently released and a lot of mini albums (character/drama cd's and what not).