Go! wasn't even published as its own project [1] until 2015, well after Go(lang) was available. It previously existed as an unpublished CVS repository in an obscure Sourceforge project [2]; I can't fault the Go developers for ignoring or overlooking it. You can find a circa-2002 version of Go! in the archived CVS repository of the Sourceforge project, but none of the project's release files appear to contain it.
(Ignore the "Last Update: 2013-09-06" on the project page - that's the date that SourceForge performed an automatic migration. Any real activity on the project seems to have petered out around 2002, with one final file released in 2003.)
Due diligence would have revealed the Go! project in a standard literature review.
I can and will fault the developers because even if they had overlooked it, they were explicitly asked, and declined to do so because they determined (themselves) there would be no confusion. So it wasn't that they overlooked it, or ignored it, they decided they were asked directly and responded "we don't care".
If the reasoning boils down to "We can do this because you are small and we are big" I cannot support that.
(Ignore the "Last Update: 2013-09-06" on the project page - that's the date that SourceForge performed an automatic migration. Any real activity on the project seems to have petered out around 2002, with one final file released in 2003.)
[1]: https://github.com/fgmccabe/go
[2]: https://sourceforge.net/projects/networkagent/