It appears that at least a substantial part of Gaertner's email to Fauci came from this 2005 academic paper:
https://www.pnas.org/content/102/33/11876
Inhibitors of cathepsin L prevent severe acute respiratory syndrome coronavirus entry
Authors: Graham Simmons, Dhaval N. Gosalia, Andrew J. Rennekamp, Jacqueline D. Reeves et al.
PNAS August 16, 2005 102 (33) 11876-11881; https://doi.org/10.1073/pnas.0505577102
The relevant passage is:
"Intervirion Fusion. HIV-luc(ACE2) (500 ng of p24) was mixed with 1,000 ng of p24 of HIV-gfp particles incorporating ASLV-A envelope, SARS-CoV S protein, or both envelopes in PBS at 4°C for 30 min to allow binding. Samples were raised to 37°C for 15 min to allow for conformational rearrangements. Virions were adjusted to the desired pH with 0.1 M citric acid. PBS, TPCK-trypsin (final concentration 10 μg/ml), CTSL, cathepsin B (CTSB) (final concentrations 2 μg/ml) or CTSL buffer alone was then added. Recombinant CTSL (R & D Systems) was preactivated by incubation for 15 min at 10 μg/ml in 50 mM Mes, pH 6.0, on ice. Recombinant CTSB (R & D Systems) was preactivated in 25 mM Mes, 5 mM DTT, pH 5.0, for 30 min at 25°C. After a 10-min incubation at 25°C, proteolysis was halted by the addition of 300 μl of DMEM10 containing leupeptin (25 μg/ml) and STI (75 μg/ml). Virions were then incubated at 37°C for 30 min to allow membrane fusion. 100 μl of the virion mixture was added in quadruplicate to HeLa-Tva cells pretreated for 1 h with leupeptin (20 μg/ml). The cells were spin-infected and incubated at 37°C for 5 h. The medium was replaced with fresh DMEM10 and the cells were assayed for luciferase activity 40 h later."
Anon is not specialized in this scientific field so can't really see if Gaertner's connection between this paper and SARS-CoV-2 is valid.