In that case you go to http://basiliskgames.com/eschalon-book-i
and purchase Eschalon Book I for 5,99$ which is the only other part which has been released so far.
There is a reason people don't want to buy games from million separate developer sites directly, but from their favorite digital portal(s). After all, I presume that's why the games are sold also on Steam.
If and when GOG releases the third part too, it makes very much sense for them to offer also the earlier parts on their site too, regardless of whether they already exist elsewhere as well.
And this is a special case, because as we know, GOG refused to sell Eschalon games long time ago. They changed their minds. I say, it's too late.
Not necessarily, if it is related to the forthcoming third part in which case it makes very much sense to offer the whole set on the site.
Either way, introducing these kinds of games doesn't seem to have prevented GOG from releasing Leisure Suit Larries, Daikatanas and System Shock 2s either. And as we have seen already, it is not anymore just GOG offering old games too, like all the Shadow Warriors, System Shock 2s, old Tomb Raiders etc. appearing on Steam too. So naturally GOG has to broaden their market as well, or perish.
Adding Eschalon 1 & 2 to GOG does not cost GOG much, so even moderate sales are easily profit.