"What is This Thing Called Love.  Sung by Gertrude Neisen ( or Neesen
). A female singer popular in the 10930s or maybe also in the late
1920s.  Did she ever record this song?  Is there an LP or CD existing?
As I remember, it was her signature song.

Howdy eddie9339-ga,

I was unable to find "What is This Thing Called Love" as recorded by
Gertrude Niessen, as it appears she went by most of the time.

Here are some snippets for you or anyone who might be working on this.

"1911 [-] Gertrude Niessen, vocalist, b. New York, NY, USA. USA, d. March
27, 1975. aka: Gertrude Niessen, Neesen, Nielsen. American motion picture
actress and singer of the 1930s and 40s.
...
In Oct. 1938, she recorded the song "La Conga" with Ernesto Lecuona's
Cuban Boys in London, England.
...
She appeared in the Broadway musical "Follow The Girls", where she sang
what became one of her biggest hits "I Want To Get Married" ...
...
Among her hit recordings were "Where Are You", and "Legalize My Name ..."


"23715 GERTRUDE NIESSEN SOONER OR LATER
23715 GERTRUDE NIESSEN THAT'S GOOD ENOUGH FOR ME"


"SINGING LADIES OF THE SILVER SCREEN (1930-up)
...
HIGHLITES: EXACTLY LIKE YOU, THERE'LL BE SOME CHANGES MADE, WHAT LOVE
HAS DONE TO ME, MY EASY RIDER'S GONE, HARLEM ON MY MIND, YOU'VE GOT ME
CRYING AGAIN, MOON SONG, I GET A KICK OUT OF YOU, I'M IN THE MOOD FOR
LOVE, Etc.

FEATURES: Ruth Etting, The Boswell Sisters, Ethel Waters, Lee Wiley,
Ethel Merman, Frances Langford, Irene Dunn, Gertrude Niessen, Martha
Raye, Dorothy Lamour, Marlene Deitrich, Kate Smith, Mae West, Helen
Morgan"

Looking Forward, denco-ga - Google Answers Researcher

eddie9339 --

I have sent an e-mail to a prominent expert on 1920-1950 jazz, and I
will let you know if it results in a lead to the recording you are
seeking.

markj-ga

eddie9339 --

I have found no evidence that a recording of "What is This Thing
Called Love" by Gertrude Niesen has even been released, at least on LP
or CD. (The determination that a 78rpm recording or an unreleased
off-air radio broadcast recording may or may not exist probably would
require reviewing specialized printed sources.)

However, I have discovered that Niesen did sing (and record) with the
Leo Reisman Orchestra for a time during the 1930s and that Cole
Porter's "What Is This Thing Called Love" was Reisman's theme song. 
Unfortunately, the only Reisman recording of that song that I have
found (and it is available on CD compilations) had a vocal by Lew
Conrad.

I will keep looking as long as your question remains open, since
pre-WW2 pop/jazz music is one of my great interests, but I am
increasingly pessimistic about finding a recording that meets your
needs.

markj-ga