Martinelli was signed from Ituano FC in Brazil on 2 July and scored in the second half of the pre-season friendly.
Bukayo Saka and James Olayinka were the other goalscorers as the Gunners began their tour of the USA.
Many of their bigger names should return for their next game with Bayern Munich in Los Angeles on Thursday.
Of the team that began in Denver, only defender Carl Jenkinson started a Premier League match last season.
But star trio Alexandre Lacazette, Pierre-Emerick Aubameyang and Mesut Ozil all came off the bench for late cameos against their MLS opponents.