Pgtcl is the name of the [Tcl] package that is made available when [libpgtcl] is built and installed.

In the past, many people have loaded libpgtcl by using an explicit load...

 load ./libpgtcl.so.14

In the modern era, the libpgtcl shared library is built and installed and automatically loaded when
the Pgtcl package is requested...

 package require Pgtcl