Allen's call up means there are now a number of players in Gatland's squad who have represented Wales on the IRB World Series circuit. Owen Williams, Justin Tipuric and Rhys Patchell have all featured for the Sevens under head coach Paul John, with Allen the latest to join them in progressing to the senior ranks.
The 20-year-old centre made his Sevens debut in Dubai last year and helped Wales to a Cup final against Fiji in Hong Kong, scoring a superb solo effort in that game. Now, he is looking forward to making an impression this autumn.
"I've played for the 18's, 20's and Sevens but the Welsh senior squad is the pinnacle of what you can achieve," said Allen.
"I can't put into words how I'm feeling at the moment - I'm still in shock but I can't wait to get training now.
"If you look at the Welsh squad, there is a high percentage of players who have played Sevens. I think it's a brilliant way to bring young players through and it's a great stepping stone between Premiership and regional rugby.
"The Sevens was the first time I was away for a long time with a group, playing against top teams week in week out on the world stage. Playing in such wide open spaces helps so much in attack and defence and it gives you a lot of confidence, especially when you have good performances like Hong Kong."
After representing Wales Sevens and Wales Under 20 last season, Allen underwent shoulder surgery in the summer, an injury which forced him to miss the Junior World Championship in France. This season, he has quickly developed into a key centre option for Blues Director of Rugby Phil Davies, forming an exciting midfield partnership with Williams.
"I came back from my shoulder operation, played a game for Cardiff and got called up quite quickly," said Allen.
"I thought I'd be with Cardiff for a couple more games but Phil said he wanted to give me a chance and he's backed me since the start of the season. He gave me some games from the bench which I think I needed to get up to speed and then a couple of starts."
Allen helped the Blues to a famous Heineken Cup defeat of defending champions Toulon last Saturday, his call up to the Wales squad completing a dream few days.
"Some of the boys had texts yesterday but I had no service on my phone. I didn't think anything of it but a couple of the boys came in and said congratulations," said Allen.
"I didn't shake their hand at first as I didn't believe them and had to go check for myself.
"It's been a pretty surreal couple of days. I haven't really got over beating Toulon yet and with this news now, it's probably been the best few days of my life.
"I'm over the moon to be in the squad but the next step is to push for the 12 jersey. If I do my best in training, we'll see what happens."