Gibbs, being Gibbs, would show his support in his own way. He might not say much but would give Tony a nod of approval. Abby would be all squeals and hugs, and she'd probably want to be involved in every detail of the wedding planning. McGee would be happy for them and might even write a special little speech for the wedding. Ducky would offer some wise words about marriage and relationships based on his years of observing people.
Well, Gibbs would have that look in his eye, a mix of pride and a little bit of 'Don't mess this up, DiNozzo.' He might also secretly make sure that all the security arrangements for the wedding are in place. Abby would be the life of the party in the wedding preparations. She'd be bouncing around with ideas for the bachelorette and bachelor parties. McGee would be the one to document the whole process, from the proposal to the big day, maybe even writing a blog about it for the NCIS team to read and enjoy.