Whilst the heartwarming is minimal in this film, the scene where Mark, upon reaching his mother's deathbed, ends up inventing religion, changing the world and its outlook on death simply so she can spend the last few moments of her life happy definitely qualifies for this.
His refusal to abuse his talents, such as getting people to sleep with him or lying about his genetics to his love interests, even if it costs him the relationship, are also quite nice.
Greg is just as plainly honest as the next person, but it becomes heartwarming when he tells Mark that he hasn't lost Anna yet.
In a meta sense, The scene where Mark invents religion to give his mother happiness on her deathbed is probably nicest thing Ricky Gervais has said about religion. He always mocks and insults religion since he is an atheist but in this scene, Gervais is being mature by saying that religion is still a lie and but a belief in an afterlife can be reassuring and somewhat comforting.