Notepad like text editor based on the Scintilla source code. Notepad3 based on code from Notepad2 and MiniPath on code from metapath. Download Notepad3:
New Lexers have to be requested for Lexilla Component.
A request has been made 14 Years ago: https://sourceforge.net/p/scintilla/feature-requests/731/
Since Lexilla author does not implement Lexers himself (with a few exceptions), it seemed that last 14 years no one voted in to implement (and care fore) a Groovy Lexer, so probably it will not be implemented in Lexilla and so not find its way to Notepad3.
Sorry.
The best you may do now is to find a Lexer, which comes close to Groovy and add Groovy's source file extensions to this Lexer's list of Associated filename extension in Lexer's scheme customization.
New Lexers have to be requested for Lexilla Component. A request has been made 14 Years ago: https://sourceforge.net/p/scintilla/feature-requests/731/ Since Lexilla author does not implement Lexers himself (with a few exceptions), it seemed that last 14 years no one voted in to implement (and care fore) a Groovy Lexer, so probably it will not be implemented in Lexilla and so not find its way to Notepad3. Sorry.