The 75-million-pound arrival from Newcastle United was introduced at half-time, shortly after Martin Baturina had levelled with an impressive finish.
Arsenal had taken the lead before the interval through Myles Lewis-Skelly, whose future has recently attracted attention amid reported interest from Chelsea and Manchester United.
Guimaraes received a rousing reception when he entered the field, with supporters chanting his name and applauding his first involvement. The Brazilian quickly demonstrated his combative qualities, producing a well-timed challenge to stop a Como break before calmly returning possession to David Raya.
He also offered glimpses of his creativity, repeatedly attempting passes over the opposition backline. One clever ball released Kai Havertz, who finished the move but saw his effort ruled out for offside.
The new signing initially partnered Lewis-Skelly before moving into the deeper midfield role following the introduction of Martin Odegaard and Mikel Merino. His display also hinted at a promising understanding with Odegaard.
Mikel Arteta had fielded an experimental side before making extensive changes ahead of Sunday's Community Shield clash with Manchester City.
Guimaraes later converted his penalty in the pre-arranged shootout. His competitive debut could come against City, with Arteta set to decide whether he is ready to start.
"Let's see. He only had three training sessions. But he looked good," Arteta said, when asked if Guimaraes will face City.
"He was really pushing to play some minutes today. I think he wanted to get his debut and the feeling of playing in front of our fans immediately.
"You sensed immediately a really strong connection and a player that is going to make us better without a doubt."
