I think the reason he was deported might be whatever is written on the top right corner of the back of the card, but, I can't read it. Gammon and Gorman were common "americanizations" of Gaumond so these alias names do not mean he was a criminal. He could have been sick, or didn't have enough money, or the reason he gave for coming may not have been acceptable. It's not clear to me if he was coming for a visit, or to immigrate, and that may be the problem. On the front of the card is the reason he was going to the US - something about a farm but I can't read that either.
The rest of it says the alien herein named deported by me to Canada May 3, 1938 via Detroit and Canada Tunnel.
In conformity with the exclusion order entered by the board of special inquiry.
There is a box on the front that says Act of 1924, so he was deported under the rules of that Act. That's the immigration act that prohibited Asians from coming to the US and also enacted quotas limiting everyone else.
You would have to find out if there are any records for the Board of Special Inquiry to find out more.