GNC, the US-headquartered vitamin and nutrition chain, has annual revenues of $2.5 billion and employs 16,500 staff across 50 countries.  But it is struggling. In 2020, weighed down by more than $900 million in debt and with sales falling, it filed for Chapter 11 bankruptcy in the US, eventually shuttering 1,200 of its 5,200 stores.  Founded in 1935, GNC was the go-to retailer for vitamins, protein powders and nutritional supplements in the US. But for nearly a decade, it has rapidly lost market share to chains like Walmart, Target, CVS and Costco. The company has 11 stores in Ireland, concentrated in…