They didn't get to spend Valentine's Day together at all.
She spent the day with Richard, feigning an interest in him whilst anxiously worrying about how Gumball felt, and if he was angry that they couldn't be together on this special day.
He spent his time hanging out with Darwin, Carrie and Penny on what was supposed to be just a friends hanging out thing but that became kind of a double date thing, which made him upset at Darwin and Carrie for trying to shove him and Penny back together after they had drifted apart.
In the end, both Gumball and Nicole get home mentally and emotionally exhausted, exasperated and upset at the people they had to spend time with, and worried out of their minds about what the other person was thinking and if they could even keep this up, and if it was worth it at all to live this lie.
But then they get their solitary moment together, half past midnight with everyone else in bed. They sneak down into the living room so they can just sit on the couch and hold each other, and all the worry and stress dissolves in that embrace. They love each other, and Valentine's Day is just like any other day. They'll always find a chance to be together when they need it. Nicole is antsy, and wants her Gummypuss right there on the couch but he cautions against it. There'll be another time for that. They always find a time and place. So they share a quick kiss and make plans for their next clandestine meeting. Then they both go to their separate beds and wish things were different. They think again about whether it's worth it to be together, with all the world against them, scuttling about in the shadows.
But they remember their first time together, and the things they've said to each other, and how this crazy thing started in the first place. They smile. It's worth it.
They could live this lie together forever.