Star back-three operator Halfpenny has made the final 31-man cut for Japan and is now preparing for what will be his second global showpiece tournament.
The 30-year-old played a key role as Wales reached the 2011 semi-finals, but he missed the 2015 edition after suffering a knee injury against Italy.
It means Halfpenny can’t wait for this year’s competition.
“I was extremely nervous in the build-up to the announcement and I just tried to keep myself busy in the morning,” Halfpenny said.
“I headed out for breakfast with my partner [Jess] and the little one [baby Lily], but when the news came that I was in I was delighted and relieved to hear my name.
“I feel for the boys who weren’t included because they worked so hard, just as everyone else did.
“It’s been a good couple of months training. It’s been extremely hard in the camps in Switzerland and Turkey. We’ve been pushed to our limits at times both mentally and physically.
“The boys have worked extremely hard, preparation has gone really well, and we have one last important game against Ireland which is crucial in the build-up to the World Cup.”
Halfpenny didn’t feature in Wales’ Guinness Six Nations Grand Slam earlier this year after taking a head knock in the November win over Australia. But he has been training fully with Gatland’s squad all summer and stepped in as a last-minute replacement to start the 13-6 World Cup warm-up win over England in Cardiff.
Wales’ last warm-up clash is away to Ireland on Saturday.
Looking ahead to a trip to Dublin’s Aviva Stadium, Halfpenny said: “We’re not sure what the team will be. The squad hasn’t been announced, but if I’m selected to play then I’ll be extremely excited to get out there and get the jersey on.
“The strength in depth we have is extremely strong. What the squad has achieved over the last 18 months has been phenomenal. We’ll take confidence from that and have the belief we can go out to Japan and really achieve something special.
“We’re going to have to go up another level to achieve that. The competition in the back three is extremely strong. It’s what you want because it only pushes you harder to improve your game.
“If we want to go out there and achieve something special we need that competition.”
Halfpenny has been a part of three British & Irish Lions tours and has 81 Wales caps to his name.
His knowhow will be vital to Gatland and Wales in the next six weeks.
“I’ve never been to Japan. I’m looking forward to seeing what it’s like and experiencing the culture. I don’t mind a bit of sushi so it will be good to try some of that too,” Halfpenny said.
“Our focus has to be on what we do and not other teams. It’s all about what goes on in our circle.
“It started when we joined up in June and continues going into this Ireland game. It’s hugely important we focus in on what we’re doing and not what others are.”