To be fair, they might have been referring to Black Cauldron with the "Disney characters" line. Also, IIRC Secret of NIMH was the first film Bluth made after leaving Disney.
Gurgi would certainly fit "Disney character", but with the Youtube link to The Secret of NIMH
and the talk about ex-Disney employees, I assumed they were talking about the Bluth film.
There was also Banjo the Woodpile Cat
, released two years before The Secret of NIMH
, but that was a 30-minute short film, and began as a side production while Bluth still worked at Disney. Pick your level of pedantry, I guess.