“I think it is impossible to copyright empty space” (Pierre H quotes Pierre M)
Actually, we did get a reply from MonoType’s lawyer, acknowledging our letter, and asking us to identify ourselves as a legal body. We responded with a long list of our names and various locations; we haven't heard since.
For metrics we used the existing Gill Sans metrics. There is some antecedent to using existing metrics. Ascender Corporation made the Liberation fonts for Red Hat match the metrics of Times New Roman, Arial and Courier New. Bosch Office Sans, by Erik Spiekermann and Christian Schwartz, matches Arial metrics. Yes, we could have been clearer about this in our letter.
With regards to quality, we are aware that using Potrace doesn’t produce the same kind of optimized Bézier curves that are produced in a traditional type design process. This is not necessarily what we are interested in. We are interested in the process, in ways of producing. Iterative, collaborative. Release early, release often. Test out ideas and processes. The difference between a traditional typeface and an open source typeface is that an open source typeface can be redistributed and modified.