Richard,

           I am going to wait on answering this on the group until after I get a definitive answer as to whether JAWS supports regular expression syntax in pattern matching for match/substitute pairs.  If it does, then discussing how regular expressions work, at least to some extent, is on-topic because it's something a JAWS user might employ.  If JAWS support this, then it's entirely off-topic for this group, though I'd be happy to discuss it privately.

           There are lots of beginners guides to regular expressions available on the web and many of them will do a better job walking folks through than I might.  

           But, because I said this earlier I'll repeat it again on the assumption that JAWS does support regular expression matching, to solve the original questioner's problem one would use:
                                 GD-(\d{1,})      or , if you don't care whether either the G or D or both are upper or lower case:  [Gg][Dd]-(\d{1,}) 
as the pattern match and:
                                 Gee Dee \1
as the substitution.

All of the above presumes you do not want the dash pronounced.  If you did then the substitution would be:  Gee Dee Dash \1
--

Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041  

The purpose of education is not to validate ignorance but to overcome it.
       ~ Lawrence Krauss