I think Gasly will do well, and RBR never had a true #2 driver. So far Gasly's season is very similar to Verstappen's first season: In the first twelve races both scored 26 points, with a fourth place being their best finish.
Gasly is also the last GP2 champion (befrore it became Formula 2), and almost won the japanese Super Formula title in his first season.
The curent 2019 line-up so far: (Confirmed, likely, not likely)
Mercedes: Hamilton, Bottas
Ferrari: Vettel, Raikkonen
Red Bull Racing: Verstappen, Gasly
Renault: Hulkenberg, Ricciardo
Haas: Magnussen, Ocon, Leclerc, Grosjean
Force India: Perez, Stroll
Toro Rosso: Hartley, Norris, Vergne, Buemi
Sauber: Ericsson, Leclerc
McLaren: Sainz, Vandoorne, Norris
Williams: Sirotkin, Kubica