Question - winning notification

After the giveaway ends it takes a few seconds to draw a winner, you can't win 1 hour later or even months unless someone has requested a reroll, which is rare.

usually a few seconds after the end, but actually can happen you win it days later in case the first winner didn't claim his win, or because gifter asked for a new winner due to first winner already own or what ever reason.

remove myself from some "less desired" GAs.

:scratches head:

when exactly I receive the winning notification (that flashing cat GIF)?

Immediately, but it doesn't always pop up as it should, or it can show up in another open Steamgifts tab, so it's easy to miss.

When GA ends, I hit F5 and do not see the gif, does that mean for 100% I did not win or can I still win that game an hour or day later?

You can just ignore the entire concept of the popup, it's not a meaningful site element. A more reliable approach is to check whether you have a red number on your win trophy icon on the top of the page. If you've won, it'll show a red 1 on it (which will start flashing once the GA creator has confirmed sending the game). The pop-up is just there for added visual flair, and to make wins feel more impactful.

Of course, as noted by others above, rerolls can still get you a win later on (which is treated the same as any other win, triggering both the icon alert and the cat pop-up), but otherwise you'll know immediately whether you won a giveaway or not just by checking against the presence (or lack of presence) of a win alert on the icon at the top of the page.