I made my estimate on a couple of observations. The 1984 catalog specifically mentions a new line and features the investment cast shell, crown and lugs of the OP's bicycle. It also shows the internal cable routing but only one set one bottle bosses. So the bicycle appears to be no older than 1984 and that assumes a running change to dual bottle bosses during the year. Yet the bicycle has pre-1988 decals, so unless Gios was not rotating it's decal stock, the frame would appear to be 1985-1987.

Maybe I'm wrong but I seem to recall Gios being around a couple of years earlier than yourself. No doubt, A Sunday in Hell brought about wider exposure and greater popularity but they had already gained quite of bit of prominence from De Vlaeminck's earlier victories in major classics such as Milan-San Remo (1973) & Paris-Roubaix (1974 & 1975).