By process of elimination, it should be a 1989 model. The 1987 version was called the Gara Triathlon and had chrome stay ends and embossed stay caps. The 1988 version was the Gara 88 and was labelled as such, using a different font. It also wasn't offered in white. The 1990 version used totally different decals and had internal cable routing for the rear brake.
Regardless, it has been frankenbiked to an extent. The saddle is obviously not OEM and the rims are mismatched, neither of them being from Miele's standard source during this era.