it is very different from the asterisk gui, it can be more considered to be a “configuration engine” - it is a one way programming flow - changes you make in the gui will be written to the files, but hand made changes in the files can never be read to the gui, this is why you cannot import your config from the asterisk gui into freepbx. But freepbx offers many more features at the click of a button, and it is very fast to work with, there are even extension import utilities, so it shouldnt take too long to re-configure it.
My recommendation is to try it in vmware to see if you like it.