Deception, disguise, and betrayal are par for the course in Two Gentlemen of Verona. When characters disguise their identities and/or their true intentions, the result is a plot with the kinds of twists and turns that we've come to expect from Shakespeare's comedies. The play is also interested in the moral and philosophical aspects of deception (although, it handles the theme with a pretty light hand). While some lovers play silly mind games, Proteus betrays his best friend and girlfriend by going after Silvia. In the play, it seems that the betrayal of male friendship is one of the worst crimes imaginable. (Strangely enough, the play also suggests that it's even worse than attempted rape.)
Although Julia's "Sebastian" disguise deceives those around her, the disguise also allows her to speak the truth about her feelings for Proteus.
By the play's end, Proteus believes that his betrayal of Valentine is the worst kind of deception there is – it's even worse than his betrayal of Julia.