PCIe x8 Gen3

PCIe slot will run at the lowest denomination speed The most common PCIe slot currently in use are PCIe x16 slots, PCIe x4

It would not be a problem if quickspecs would mention in any way that the PCIE slot 3 of DL360 Gen 10 has limited compatibility However HPE states that PCIE

pcie 20 x16 slot PCIe x8 Gen expansion backplane with one PCIe slot The PCIe x8 Gen3 backplane supports a PCIe Gen3 target cable adapter installed in the OSS

pcie 4 slot TL;DR: If you really want that 6750XT it should work fine in a slot My PCIe motherboard supports ReBAR so not all Gen 3 motherboards