'it is generally well-known that gifts/free things are never really "free"'

Although for some definition of altruism, this could always be argued to be true, I think this misses some nuances.

If you give to charity (say with motivation being a relief from guilt or a feeling of social duty), I think most people would consider that an altruistic action. In that situation, you have no real expectation of future return, so I would say the gift was "free".

Or similarly, when you care for a dying relative, there is often no expectation of benefit other than from the act of giving itself.