Like Ian's adoptive father Frank Gallagher, Gary is an alcoholic and a father of many.
During the 1980s, Monica worked at an off license and befriended Gary. She confided in Gary about her problems with her husband Frank and they eventually ended up sleeping together; nine months later Ian was born. Monica chose not to tell Ian that his real father was Gary, and kept it a secret from her family, with Gary also unaware he had another son.
When he was 15, Ian discovered the truth about Gary and decided to track him down, thinking that maybe he was a good father. He and his brother Lip tracked Gary down to his local pub where they found him in a drunken state. Ian chose not to contact Gary and thereafter considered Frank as his father.
In Series 5, Ian was horrified to discover that he was dating Gary's son, Sean. Monica discovered Gary had died recently and went to his funeral to claim something that she had given him. Ian came along with her and discovered Sean was his brother. Sean didn't see their relationship as a problem, stating that as no one else was aware and they couldn't have children together there was no real reason why they should break up. Ian liked Sean greatly but the idea of being in a romantic relationship and having sex with his brother was too horrible to him and the couple split.