this is the info that attracted my attention to the reappraisal literature:

So, to prove the existence of a retrovirus one is obliged to:

1. Culture putatively infected cells.

2. Purify a sample in a sucrose density gradient.

3. Photograph the 1.16 band proving there are particles of the right size and form, and there is no other material.

4. Extract and analyse the constituents of the particles and prove they contain reverse transcriptase by showing they can make DNA from RNA.

5. Culture purified particles with virgin cells demonstrating that a new set of particles appears with the same morphology and constituents as the originals.

Gallo didn't do that.....