Advice on porting to Macintosh

I have a small (500 lines) utility that I have written in Gnu Pascal 
under Linux for flashing the software on a PVR via a serial port (which 
can be provided by a USB to serial converter where necessary).  I want 
to port this software for use on Apple Macs (both Power PC and Intel) 
under OS X.  I have access to an Apple eMac running OS X 10.3.  I know 
next to nothing about OS X.  What compiler would be best?  If I want to 
add a simple GUI to select a file what would be the easiest way?  Any 
other advice?
-- 
Martin Liddle, Tynemouth Computer Services, 3 Kentmere Way,
Staveley, Chesterfield, Derbyshire, S43 3TW.
Web site: <http://www.tynecomp.co.uk>.