The following is about Genius, which has now been merged with drgenius. Look at the Dr. Genius Home Page for more information. So the following is not absolutely correct and mostly refers to the genius part of Dr.
Genius is a calculator program similiar in some aspects to bc and matlab.
GEL is the name of it's extention language, in fact, a large part of the standard genius functions are written in gel itself. Why another programming language. Because I don't think any of the current programming languages do math justice. And matlab requires one to make a new file for each new function. Something had to be done.