De is a partitive article. If you had a different noun you could have :

Ce qu’on appelle :

• de la rage
• des hommes avisés

ce que l’on appelle de l’amour

is what I would expect here in the context of Gide’s book.

We could of course encounter the phrase ce qu’on appelle amour. For example, Nietzsche’s text Was Alles Liebe genannt wird has been translated as Ce qu’on apelle amour (The things people call love).

In Gide’s text we are dealing with a specific case with particular people involved. Nietzsche’s text is philosophical and deals with love as a (philosophical) concept.

We could also have ce qu’on appelle l’amour. I find it difficult to see a difference in the case of Gide’s sentence.

I’m fully aware it’s beside the point, but it’s just occurred to me that in English you would not differentiate between those, unless maybe you capitalized Love… ?