I found this site last week because it appeared on the all sites menu on one of the other stackexchange sites I was using at the time. Now it seems it is no longer listed on the all sites menu on any of the other stackexchange sites I use. What gives?
It is present in the all sites menu right between Philosophy and Travel, if you sort by oldest.
Were you looking some where else? It could be possible that you saw an advertisement for the site and these appear randomly and cannot be predicted.
In response to your comment below:
Clicking on the all sites tab in the StackExchange menu, I still see this site third from the top.
I'm not entirely sure, but I believe that the position of a site in the list depends on the rep you have on those sites (>200 rep). For e.g., among the sites I have >200 rep, I have the most rep on SO followed by MSO, then G&L, Math and Cooking and that's the order I see it displayed. After these, it displays the rest of the network sites in presumably random order. I tried this out from multiple SE sites and I'm pretty sure this is the case.
So since you have <200 rep on G&L, what must have happened was that it got shuffled in the list and you just didn't scroll down far enough to see it.
If you access the StackExchange menu from a site where you don't have an account (e.g, programmers.SE for me), the ordering is random. Nevertheless, all the sites are still displayed.