Fr. Gabriel often visited people who had different religious views. He tried to share grace with everyone and lead them to the true faith. He says himself:
"Once, dressed as I was in my monastic schema, I went to the Synagogue. At the entrance hung a large portrait of Moses. I venerated it. The attendees liked this. Moses was a great prophet and I gave him his due respect. I had entered as they were reading from the Bible. Few were there and they looked at me with surprise. I began to preach, and when they heard about Christ they called for their High Priest. He came out but he did not stop me. Afterwards he invited me into his office. He listened with great interest and attention to what I said about Christ. Finally, he thanked me and we bid farewell.
I also went to a Mosque. Outside were a group of Muslims sitting and drinking tea. When they saw me they stood up and respectfully invited me to their table. There I preached only love and we parted sweetly.
I went to the Baptists as well, but they did not receive me. When they saw me, they became irritated, calling out for their pastor to expel me."
When Fr. Gabriel spoke about those in heresies or believers in other religions, he always stressed that we need to show the same love to all.
A nun told him once:
"I spoke with a heretic. I became so enraged when he cursed at me, that I felt like I had sinned."
Fr. Gabriel sweetly replied to her:
"Your heart is not a stone so as to listen with indifference when someone curses at you for your faith. If you are not as innocent and harmless as a little child you will not enter the Kingdom of Heaven. But that does not mean you should have the mind of a small child!"
To another nun he said:
"There will come a time when you will be led by worldly people. I hope that a heretic or a Baptist or a Jehovah's Witness comes to me. I will seat them, feed them and teach them the good word. I will not sit with atheists. If one comes, do not get up for them. Do not give them your seat. Let them sit by themselves. You did not go to them, but they came to sit with the faithful. Will all stand for him?"
Source: From the book Saint Gabriel the Fool for Christ and Confessor [Greek]. Translated by John Sanidopoulos.