If you encounter a 504 Gateway Timeout error at 99% of the installation prerequisites, the issue may not actually be with VxRail itself. Instead, check the vCenter server task list and look for the vLCM task “Sync updates”

If this task takes longer than 10 minutes, that’s likely the problem. VxRail launches this task during its prerequisite checks and waits for it to finish—but it only waits for 10 minutes by default. This is a documented issue by Dell, and their recommended workaround is to raise the timeout to 20 minutes.
(Dell KB link: https://www.dell.com/support/kbdoc/en-us/000204593
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That works fine—until the Sync Updates task takes more than 20 minutes. I’ve seen this happen, and even if you try to raise the timeout to 30 minutes, you’ll find it still stops at 20 due to a hard-coded limit.

My solution is simple—and easier than Dell’s SSH method.
Rather than extending the timeout, make the Sync Updates task complete faster than 20 minutes (or 10 minutes if you skip Dell’s workaround):

Go to the vLCM page in vCenter → Settings → Patch Setup.

Disable all download sources containing *vmware.com* in the URL (these are no longer valid and cannot be removed at the time of writing).

Disable all Broadcom-related sources as well. (Don’t delete them—just disable.)

After these changes, the Sync Updates task should finish in just a few minutes, allowing your VxRail upgrade to proceed without timeout errors.

Finally, don’t forget to re-enable your sources afterward (turn the blue ones back on; yellow can remain disabled).