Scott Pakin's Comprehensive LaTeX Symbol List identifies the structure of extensible delimiters in section 8.2 Resizing symbols (p 100-102):
All variable-sized delimiters are defined (by the corresponding .tfm file) in terms of up to five segments, as illustrated by Figure 1 on page 102. The top, middle, and bottom segments are of a fixed size. The top-middle and middle-bottom segments (which are constrained to be the same character) are repeated as many times as necessary to achieve the desired height.
This is merely informative since I am unfamiliar with the procedure of how to create these things in a .tfm file.
Conveniently, scalerel provides \scaleleftright[<max width>]{<left obj>}{<stuff>}{<right obj>} (and a comparable \stretchleftright) for scaling/stretching both <left obj> and <right obj> to the height of <stuff> (constrained, if required and optional, to a width of <max width>). Here's a quick example:
Best Answer
A Gamma is an upside down L: