Who first used the corner quotes, ⌜ and ⌝, or $\texttt{\Godelnum}$ with Sam Buss's macro, for the notion of Gödel number?
Quine introduced corner quotes, but did not use them for the notion of Gödel numbers.
The corner quotes were used for Gödel numbers by Joseph R. Shoenfield, Mathematical Logic, 1967, as from page 122.
The Corner quotes are used prevalently in provability logic, and in other areas of logic. Two important citations are Craig Smorynski, The Incompleteness Theorems, in Handbook of Mathematical Logic, as from 1st edition 1979; and Paanu Raatikainen, Gödel's incompleteness theorems, in Stanford Encyclopedia of Philosophy.
Edit: I do not find the corner quotes in Martin Davies, The Undecidable, or in the books by Moztowski or Kleene.