Express Engine can be run as a GUI application or as a command line application. Currently, development is done using Xanalys LispWorks on Linux. The GUI version was written using LispWorks' CAPI system so only LispWorks can build the GUI at this time.
The application is currently undergoing massive changes part of which is a switch to a completely CLIM based GUI. Hopefully, this will mean that CMUCL or SBCL combined with McCLIM will be a viable development platform.