He always hears the guests before they come, snatches of conversation and hoofbeats carrying to his ears despite the silence. He doesn't know how, and he doesn't really care; it means he can inform his mother, and there will be a hot meal ready when the travelers arrive.
Gwydion never thinks about his gift. It's just there.
He catches the thoughts of others, sometimes. He knows of Agravaine's disdain, but disregards it. He knows Gawaine's shame at his insecurities and wonders at it, at Gawaine who is strong and brave and skilled and...afraid? Gaheris is the only one Gwydion can't read, but Gareth is an open book, and Gareth always knows Gaheris's thoughts. Gwydion is acutely aware of Lot's loathing. For a long time he feels that shame, more poignantly than any beating, but it grows into hate and then numbness. Gwydion is scalded by Morgause's private impatience, swamped by her mania of twisted plots. Perhaps more than anyone else, he is aware of the cool calculations beneath her faux-affection.
Gwydion never thinks about his ability. It's useful knowledge, and he hides it.
He adjusts his actions to the moods of others, sees through their guises. Yet having knowledge doesn't always mean he has a choice; he knows Morgause feeds him filtered fairy tales, to make him share her distorted reality… But she offers the illusion of love, so that Gwydion, for all he knows they are lies, is tempted to believe them.
Morgause turns everyone like little keys in little locks, uses what she can of everyone she meets, and if she can find nothing to use disregards. Her children are little coins that can be used to buy different services. Agravain buys her violence for her sake. Gaheris she manipulates for the sport of it, some sick satisfaction. Gareth loves her. Gawain is bravery and honor. Mordred is revenge.
He knows full well he's being controlled; by now, it's obvious his mother gets what she wants.
Yet the day he sees Gareth lifeless, because of him, he wishes it were him. In a way, it is. The day Mordred faces his father, knowing they will meet their fates, he feels nothing.
He hears his father's desperate prayers, knows Arthur's fear and confusion, always confusion. He ignores it. Mechanically, uncaring, Mordred fights and knows it is for nothing
Mordred never thinks about his curse. He just hates it.