Not sure about the browsers of that era, but modern browsers will happily display ".gif" (without a file name) or even "1" (without the .gif extension).
It was t.gif, for transparent.
And it had to be transparent, because that was the only way you wouldn't see it when you added it to a table cell so the background color would show up.
Ours was called 0.gif
Clever - then you don't need to waste a whole byte for an ascii character!