That's Perl's syntax, from about 15 years before Groovy, whippersnapper. Off my lawn, now.
Oh, although I guess Perl got that syntax from the Bourne shell…
Huh, I could have sworn that I remembered that Groovy has taken it from Ruby.
Incidentally, Ruby uses just # for variables, it uses #{} for arbitrary expressions. A common style prefers #{} even where # alone works, though.
An early beta version of Groovy 1.0 had it (thanks to a Sam Pullara), but it was later yanked out. Groovy's self-styled Project Manager at the time said he only wanted syntax in Groovy that would cause the Java syntax highlight rules in Eclipse and Netbeans to highlight Groovy code similar to Java, so if a manager was walking around the programming area, the screens would look like the programmers were using Java.
And that's how Groovy got its deficient string syntax.
I think Groovy's syntax is great, where quotation marks denote an interpolated string, and apostrophes denote a plain old string.
But these are nitpicks, and it's really too late to implement something like that.