One of the biggest companies that originated in wales that I've found is Cohort Studios. Cohort Studios have worked on gaming titles on the PS2 and PS3. With one of the biggest games they have helped on is Burnout Paradise which was a very successful game created by EA studios. One of Cohorts most recent games has been nominated for a Welsh BAFTA. Cohort have created of their own mobile and console games which have been quite successful. You can find out about them here:
Wales Interactive is another one of the biggest welsh gaming companies. They have just recently released a game on the 2 major consoles, Xbox 1 and PS4. It is called knee deep and the company teamed up with an american indie games company to develop it. Wales Interactive have also released a VR game which has become quite successful. Wales interactive won a Welsh BAFTA in 2013 and 2016. Welsh Interactive are currently looking for some new members to develop more amazing games, you can find out more about them here, go check them out!
There are a lot of gaming jobs available in Wales. As I mentioned massive worldwide companies are based here in Wales, so there are more Welsh people working for these companies than you think. Many companies are trying to put Wales on the map for gaming and game developing. Most companies in Wales are hiring young trainee game developers and producers to try and expand the Gaming industry. The gaming and technology industry is massively growing worldwide nearly everyday, some people say Wales doesn't have much to offer. A lot of people like myself want that to change. One day I really hope that Wales can be an major country for gaming.