Joyce is very careful to present to us Gabriel's character as a man who is defined by indecision and worry. He is introduced as he tries to charm Lily, but fails drastically. He endlessly concerns himself with how he is going to be perceived by those around him, and debates whether he should include a literary reference in his speech or not in case it will make his New Year's speech too snobbish. He is unsure of how to respond to Miss Ivor's jibes and wants to make romantic advances to his wife, but does not know the best way to go about it. In short, he is so focused on appearances and his own neuroses that he forgets to really live in the full heat and passion of the moment.
What changes him at the end of the novel is the epiphany he experiences after hearing about his alter ego, Michel Furey, who, as his name suggests, lived life in its fullest sense. Note what Gabriel says to himself as he watches his sleeping wife:
One by one they were all becoming shades. Better to pass boldly into that other world, in the full glory of some passion, than fade and whither dismally with age. He thought of how she who lay beside him had locked in her heart for so many years that image of her lover's eyes when he had told her that he did not wish to live.
Gabriel suddenly realises that he has never really loved at all, nor has he lived life fully. He is one of those who, unless something changes, is doomed to "fade and whither dismally with age." He is granted the double-edged sword of self-knowledge, and is changed utterly as a result of this self-realisation.