As announced on @banksygrossdomesticproduct on 11 December:
Photograph: banksygrossdomesticproduct/Instagram
The T-shirts are only available for sale at an event in Bristol on 12 December. All of the proceeds go to four different homeless charities.
Banksy has collaborated with a number of NGOs during the years. One of them is, of course, the @lovewelcomes project, which among other things, made the coveted “Welcome Mat”, sold at the Gross Domestic Product. 
“Love Welcomes is a creative social enterprise that helps refugee women begin to stitch their lives back together.” (https://lovewelcomes.org/pages/our-story)
Plenty of meaningful Christmas gifts at http://lovewelcomes.org.
Banksy unofficial 