Currently booked on a delta flight that departs in a little over 26 hours. Delta Gold and my reservation says upgrade requested. Why would my name NOT appear on the upgrade/standby list?

You need to check in to your flight, which can only be done 24 hours prior to departure.

If it's a little over 26 hours then I'm assuming you haven't checked in yet (unless you're talking about a later connecting flight). You won't see your name on the list until you check in.

The list you see is the airport upgrade list. Till checkin you're not on it. It is only used within 3 hours of departure. Till then the automated system keeps trying to process.

If there are other people already on the list, does that mean they have checked in for an earlier flight that is connecting to mine?

Yep, exactly. Also as people continue to check in, your position on the list may drop.

at the gate, if your name doesn't appear on the upgrade monitor, can it also be that you're so far down that there is no more room on the monitor to display those of little hope.?

What is the difference between airport upgrade list and complimentary upgrade list?

I've the same issue as OP but additionally not able to check-in even after its opened because my international leg requires airport agent document verification. I am a Platinum Member (recently through a status much, so kind of dumb on DL). Will the system continue to try to upgrade me to D1 in the priority order even if I am not checked-in?

The day of departure language is confusing to me if it means at 12am on the same date or 24 hours before departure or 3 hours before departure at the airport and on top of this, if it requires to be checked-in.

For most purposes I don't think there's a difference, just that you won't be visible on the list you see in the app or on the web site (which is also what the gate will go by, barring acts of shena) until you check-in. But prior to the gate taking control, if RM opens up additional OU space then the system will continue to upgrade those who are eligible, in order.

If you mean a waitlisted GUC on the international leg, or to D1 on a connecting domestic flight, then you don't need to worry -- failure to check-in does not inhibit upgrades (although if you haven't checked in by the time the gate takes control then they might skip over you then).

If you mean a complimentary upgrade on the international flight, there are no free upgrades to D1 or PS, only to C+.

EDIT: after seeing your post in the other thread, SFO-ATL does not have D1 service; the flights operated with those aircraft will have the lieflat seat product but the service will be domestic F in terms of drinks and the sad meal boxes. For upgrade purposes this is considered domestic first class.

Not sure about the timing either but I think it's 24h.