Gasly signed from Toro Rosso as Daniel Ricciardo's replacement at the start of this season, but is yet to score a podium finish with Red Bull, despite his team-mate Max Verstappen having scored two race wins in the last three grands prix.
Marko is not one to hide how he's feeling and he is usually brutally honest, and this hasn't changed in his assessment of Gasly.
"In (German Grand Prix) qualifying, he showed a strong reaction to his accident," Marko said to Auto Motor und Sport about Gasly, when he hit the barrier and damaged his car in practice.
"His pace in the race wasn't bad, but Gasly is just too weak when fighting and overtaking. The crash with (Alex) Albon was completely unnecessary."
Red Bull is likely to keep the same driver line-up for the remainder of the season, but has been rumoured to be considering one of the Toro Rosso drivers; Albon or Daniil Kvyat for Gasly's seat next campaign.