Gravitons are hypothetical. We don’t have any measurable evidence that they exist at all.
But, black holes do lose energy and mass from emissions! https://en.wikipedia.org/wiki/Hawking_radiation
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Until we find them, and understand how it works, the best description of gravity is given by relativity: a deformation of spacetime’s fabric.
Also, in physics there is something weird about the exact way the 4 force fields are defined mathematically: they have particles - photons, gluons, higgs, etc. - but the particles are not exactly the same as the functioning of the force field itself! Two magnets get attracted to each other, but you are not going to see any photons moving between them back and forth. The Higgs boson is somehow instrumental to the mass of all matter, yet it itself is so heavy it took a gigantic LHC just to generate a few. There aren’t any loose Higgs bosons flying around for you to catch, as far as I understand.
Whatever happens to gravitons (still hypothetical, not even a theory) and the field of gravity, they would probably behave the same way. You would need to expertfully understand the exact mathematical description to understand the difference between the particle and the field.
Old question, older answer from PBS Space Time: https://youtu.be/cDQZXvplXKA
