The ending of What Happened to Goodbye wraps things up pretty nicely for the reader, letting us know that Mclean ends up staying in Lakeview after all (yay) and graduating with her friends. It gives us a brief summary of what she's doing now, which includes working at the restaurant and dating Dave:
For once, I was really part of a class, able to partake in rituals like senior skip day and yearbook distribution, my time at a school ending when everyone else's did. I studied for finals with Dave on his living room couch, him reading up on advanced physics, while I struggled with trigonometry. Then, while he worked, I pulled cram sessions at FrayBake with Heather, Riley, and Ellis, powered all around by Procrastinator's Specials he made personally. (18.33)
The last image of the novel is of Mclean getting a snapshot from Dave on her phone; in it, he's finally made it to Texas:
There were no words, just a picture. I clicked it, watching as it filled up the screen. The shot was four hands, two with circle tattoos, all wearing Gerts. Behind them, blue sky and a sign: WELCOME TO TEXAS. (18.51)
His picture of Texas sums up everything about the journey that he and Mclean have gone on to discover themselves—they've finally made it and are secure in knowing who they are and what they want to do with their lives.