Good question Z. It looks like the guys at Strat-O-Matic have assumed that Gosselin did not make the opening day roster. And they also had Austin Davis pitch on opening day - maybe they've adjusted their 26-man roster since it was announced he was sent down later that day. Here is what I assume their 26-man roster is based on who's been in their boxscores (I have italicized those yet to appear on a scoresheet).
Catchers: Realmuto, Knapp (2)
Infielders: Hoskins, Kingery, Gregorius, Segura, Harrison, Walker, Forsythe (7)
Outfielders: Harper, Bruce, Haseley, Quinn (4)
Starters: Nola, Wheeler, Arrieta, Eflin, Velasquez (5)
Relievers: Neris, Pivetta, D Guerra, A Davis, Alvarez, Liriano, Morgan, Suarez (8)
Of that presumed 26-man opening day roster, Liriano, Harrison, Walker, and Forsythe are not currently on the Phils 40-man roster.
There are some candidates for the 60-day injured list if the season started as planned on March 26th, e.g., McCutchen and Dominguez. Maybe you could add Hunter and Arano to that list as well to get the 4 non-roster invitees onto the 40-man roster. David Robertson is already on the 60-day IL per mlb.com