I'm PRETTY sure it's the same person, since the birth date is a match - at any rate, the illustrator did live in NYC.

https://www.dignitymemorial.com/obituaries/new-york-ny/judith-brown-10474600

It's odd that I can't find any other death notice, though.

https://www.goodreads.com/author/show/204579.Judith_Gwyn_Brown
(reader reviews)

https://www.google.com/search?q=%22judith+gwyn+brown%22&source=lnms&tbm=vid&sa=X&ved=2ahUKEwidnO3rttL9AhUOkokEHQw0DZ0Q_AUoA3oECAEQBQ&biw=1920&bih=925&dpr=1
(videos - unfortunately, the most relevant one seems to have disappeared)

Most of what I posted in 2013:

http://www.lib.usm.edu/legacy/degrum/public_html/html/research/findaids/brownjud.htm

"Among the many famous children's authors whose works she has illustrated are Padraic Colum, Margaret Hodges, Elizabeth Coatsworth, Noel Streatfeild, and Andre Norton."

(Other books she's illustrated include "Amy Moves in" by Marilyn Sachs and "Mandy" by Julie Edwards.)

https://www.google.com/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1440&bih=805&q=%22judith+gwyn%22+brown&oq=%22judith+gwyn%22+brown&gs_l=img.3..0i24.1032.1032.0.1420.1.1.0.0.0.0.65.65.1.1.0....0...1ac.1.28.img..0.1.64.WORIDRCzDBc
(book covers)

WRITINGS BY THE AUTHOR:

JUVENILES; SELF-ILLUSTRATED
•Max and the Truffle Pig, Abingdon, 1963.
•The Happy Voyage (Junior Literary Guild selection), Macmillan, 1965.
•Muffin, Abelard, 1972.
•Alphabet Dreams, Prentice-Hall, 1976.
•The Mask of the Dancing Princess, Atheneum, 1989.

ILLUSTRATOR
•L. J. Bragdon, It's Fun to Speak French, Abingdon, 1962.
•Marjorie Paradis, Mr. DeLuca's Horse, Atheneum, 1962.
•Katherine Dougherty, A Street of Churches, Abingdon, 1962.
•Margaret Hodges, The Secret in the Woods, Dial, 1963.
•Marilyn Sachs, Amy Moves In, Doubleday, 1964.
•Shyrle Hacker, The Mystery of the Swan Ballet, F. Watts, 1965.
•Dale B. Carlson, editor, The Brainstormers, Doubleday, 1966.
•Scott Corbett, Pippa Passes, Holt, 1966.
•Padraic Colum, The Stone of Victory, And Other Tales, McGraw, 1966..
•Helen M. Miller, Janey and Friends, Doubleday, 1967.
•Eileen Rosenbaum, Two for Trouble, Doubleday, 1967.
•Beatrice C. Brown, Jonathan Bing, Lothrop, 1968.
•Helene Hanff, Butch Elects a Mayor, Parents' Magazine Press, 1969.

•Mira Lobe, The Grandma in the Apple Tree, McGraw, 1970.
•Mary Cockett, Rosanna the Goat, Bobbs-Merrill, 1970.
•Daniel Cohen, Ancient Monuments and How They Were Built, McGraw, 1971.
•Julie Edwards, Mandy, Harper, 1971.
•Barbara Robinson, The Best Christmas Pageant Ever, Harper, 1972.
•Wilma Yeo, The Mystery of the Third Twin, Simon & Schuster, 1972.
•Elizabeth Coatsworth, Daisy, Macmillan, 1973.
•Wylly Folk St. John, The Secret of the Seven Crows, Viking, 1973.
•Joan Tate, Ben and Annie, Doubleday, 1974.
•Patricia Clapp, King of the Dollhouse, Lothrop, 1974.
•Andre Norton, Lavender-Green Magic, Crowell, 1974.
•Marden Dahlstedt, Shadow of the Lighthouse, Coward, 1974.
•Noel Streatfeild, When the Siren Wailed, Collins, 1974, published as When the Sirens Wailed, Random House, 1976.
•Marjorie F. Stover, Chad and the Elephant Engine, Atheneum, 1975.
•Miriam Schlein, The Girl Who Would Rather Climb Trees, Harcourt, 1975.
•Anatolii G. Aleksin, My Brother Plays the Clarinet, Walck, 1975.
•Mary E. Robertson, Jemimalee, McGraw, 1977.
•S. E. Moore, Secret Island, Four Winds, 1977.
•Ida DeLage, ABC Santa Claus, Garard, 1978.
•M. Schlein, I Hate It, Whitman Publishing, 1978.
•John Bellairs, The Treasure of Alpheus Winterborn, Harcourt, 1978.
•Jack Lovejoy, The Rebel Witch, Lothrop, 1978.
•Daniel Curley, Hilarion, Houghton, 1979.
•Jan Mark, Under the Autumn Garden, Crowell, 1979.

•Lucile W. Ellison, Butter on Both Sides, Scribner, 1980.
•Audrey Friedman Marcus, But This Night Is Different, Union of Hebrew Congregations, 1980.
•Helen Cresswell, The Piemakers, Macmillan, 1981.
•Lesbia Scott, I Sing a Song of the Saints of God, Seabury, 1981.
•Lucile W. Ellison, The Tie That Binds, Scribner, 1982.
•A Window to Look Through, Scribner, 1982.
•Joanna Cole, A New Treasury of Children's Poetry: Old Favorites and New Discoveries, Doubleday, 1984.
•Elizabeth Sachs, Shyster, Atheneum, 1985.
•Miss Cecilia's Heart's Desire, Crown, 1986.
•Denise P. Fox, Through Tempest Trails, Atheneum, 1987.
•Norma Tadlock Johnson, Bats on the Bedstead, Houghton Mifflin, 1987.
•Phyllis Reynolds Naylor with others, Maudie in the Middle, Atheneum, 1988.

•Herbert G. Draesel, Bless All Creatures Here Below: A Celebration for the Blessing of the Animals, Morehouse, 1997.