It would appear – as at Summer 2024 – that the great chamber is not available to view internally except upon special ‘guided tour events’ – check out this page for details:
cadw.gov.wales/visit/places-to-visit/barclodiad-y-gawres-chambered-tomb#opening-times
Although the cave was discovered a couple of years back by female cavers slim enough to fit in the tunnel where at least 15 skeletons were interred... women far braver than I’ll ever be... I only read about this tonight in The Tmes. And already it’s on the Wiki.