http://www5f.biglobe.ne.jp/~hankotu/etc.htm
http://www.critical.ne.jp/~kaze/gm/gm_1990.html
Both pages list ズーム・リバーシブル・バンド, or "Zoom Reversible Band" as the composer (search for ラグーン).
http://page11.auctions.yahoo.co.jp/j...9?u=iibinbokun
If you look at the top picture you can indeed just make out ズーム・リバーシブル・バンド in the credits. A portion of the tracks are arranged by someone else, but at least we've got a lead.
Edit: This should teach me to actually look at the covers section first
Composed by Zoom Reversible Band
1~5: Arranged by Masae Sakaide
6~36: Arranged by Zoom Reversible Band
Performed by Zoom Reversible Band
There you have it.