Sale of guarantees of origin for electricity produced from renewable energy sources is a supply of services. Revenues from such sales should be classified as obtained from sources other than capital gains.
Sales of guarantees of origin (RE-GOs) can be considered by some as transactions concerning goods. Such train of thought, in turn, might lead them to think they are supplies of goods. As practice shows, it is quite the contrary.
For more on this topic, see an article by legal consultant Krzysztof Dżugaj in Rzeczpospolita on-line and Dobra Firma weekly.