On the site (great site by the way), it is stated that the GSTNet plugin works with A6.xx. That might suggest that the plugin works with all versions of A6, but from experience with Conitec SDK changes, I assume it does not.

The issue with Win98 compatibility probably isn't related to the GSTNet plugin directly. It seems all engine versions after A6.40.5 may be incompatible with Win98 Systems.
No Win98 for 6.60, WHY? Thread