Is trimethoprim/sulfamethoxazole, or trimethoprim by itself, commonly
used or recommended to doctors in treating gastrointestinal infections
or wounds that have become infected?  Or is
trimethoprim/sulfamethoxazole commonly used in hospitals and given to
patients involved in trauma?

This medication, commonly known as Bactrim or Septra, is an antibiotic
most commonly used in treating urinary tract infections but it is
useful against a wide range of bacterial infections.  It works in
combination, so trimethoprim alone is generally not used or available.
 It would probably not be the first choice in wound infections, GI
infection, or trauma patients, but this would vary widely depending on
circumstance.

Septra gets more use in those circumstances than you might have
thought - a trauma patient/GI infection/Wound infection with
coexisting UTI is commonly prescribed Septra if there is no other
suspected infection or if the antibiotic coverage of the primary
infection has been narrowed by sensitivity testing to the point that
it would not cover a coexisting UTI.

Also, at least here in North Carolina, a fair amount of the MRSA
(methacillin resistant staph aureus) is only sensitive to Septra or
Vancomycin.  Being that Septra is available orally, is much, much
cheaper, and reduces the risk of developing vancomycin resistant MRSA,
it is getting a fair amount of use in treating MRSA positive patients
(who could fit into either of those categories mentioned above.)

Trimethoprim as sole therapy is really only seen in uncomplicated
UTI's.  The combination of sulfamethoxazole extends the spectrum of
the drug considerably, but is not always needed for UTI's.

According to this document, TMP-SMX is being used in antimicrobial
therapy for a variety of foodborne pathogens -- primarily in pediatric
and immunocompromised patients.
http://www.cidrap.umn.edu/cidrap/content/fs/food-disease/gastro-treat/tx-gastro.html