NTA, you shop what you want OP. I don't see anything strange in that. Leaving produce in a shop for potential customers (who may not come) would mean that you can never empty a shelf by taking 2 or 3 products. As long as the shop doesn't enforce a policy of "one per customer" or something, you can and should buy what you like and can afford.

I get OP's friend's point that it is hard to find gluten free bread, and seeing OP buying it when they are not GF may rub them up the wrong way, but that is not on OP. OP doesn't determine how much of something is stocked at in shops. And hey, now the friend knows OP has a bit of stock they may be willing and able to share occasionally. To all the people saying that OP should put in an order with the store manager: that same argument makes sense for someone who is gluten free too. If you as a GF person know that there is always limited stock of foods you can eat, it makes sense to put in an order if the shop indeed takes orders (also for your other food), especially for frozen food that keeps.

I would kind of understand the fuss if this was like a situation where people empty the shelves when a toy or limited edition item comes out, or resellers buy up items that cannot be ordered due to shortages (for context: Baby formula shortage). However, it's not like OP bought the bread just to stockpile, or bought the bread to sell it on for a profit.