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