As far as I know the item specifics won’t (can’t) be set by GarageSale when adding a Product. Instead eBay will add the matching specifics to the uploaded listing automatically if „Include additional information“ is enabled in GarageSale’s Product panel.
However, it seems you still have to define the „required“ item specifics in GarageSale in order to start the listing.
For that it’s helpful to select „Insert required attributes“ in the GarageSale preferences > eBay > Advanced > Attributes Panel.
If you then open the attributes panel (if no attributes have been defined yet), only the required item specifics will be added and you only have to define those. The rest will be done by eBay afaik.