It fails at here:

/bin/sh /wrkdirs/usr/ports/devel/gdb/work-py37/gdb-9.1/gdb/../mkinstalldirs nat/.deps
mkdir nat
mkdir nat/.deps
  CXX    gdb.o
  CXX    aarch32-tdep.o
  CXX    aarch64-fbsd-kern.o
  CXX    aarch64-fbsd-tdep.o
  CXX    aarch64-linux-tdep.o
  CXX    aarch64-newlib-tdep.o
  CXX    aarch64-ravenscar-thread.o
  CXX    aarch64-tdep.o
  CXX    ada-exp.o
  CXX    ada-lang.o
...
  CXX    gdbsupport/xml-utils.o
  CXX    gdbtypes.o
  CXX    glibc-tdep.o
  CXX    gnu-v2-abi.o
  CXX    gnu-v3-abi.o
  CXX    go-exp.o
  CXX    go-lang.o
  CXX    go-typeprint.o
  CXX    go-valprint.o
  CXX    guile/guile.o
  CXX    guile/scm-arch.o
  CXX    guile/scm-auto-load.o
  CXX    guile/scm-block.o
  CXX    guile/scm-breakpoint.o
  CXX    guile/scm-cmd.o
  CXX    guile/scm-disasm.o
  CXX    guile/scm-exception.o
  CXX    guile/scm-frame.o
  CXX    guile/scm-gsmob.o
  CXX    guile/scm-iterator.o
  CXX    guile/scm-lazy-string.o
  CXX    guile/scm-math.o
  CXX    guile/scm-objfile.o
  CXX    guile/scm-param.o
  CXX    guile/scm-ports.o
/wrkdirs/usr/ports/devel/gdb/work-py37/gdb-9.1/gdb/guile/scm-ports.c:314:3: error: use of undeclared identifier 'TRY'
  TRY
  ^
1 error generated.
gmake[3]: *** [Makefile:1667: guile/scm-ports.o] Error 1
gmake[3]: Leaving directory '/wrkdirs/usr/ports/devel/gdb/work-py37/.build/gdb'
gmake[2]: *** [Makefile:8273: all-gdb] Error 2
gmake[2]: Leaving directory '/wrkdirs/usr/ports/devel/gdb/work-py37/.build'
gmake[1]: *** [Makefile:851: all] Error 2
gmake[1]: Leaving directory '/wrkdirs/usr/ports/devel/gdb/work-py37/.build'
*** Error code 1

Stop.
make: stopped in /usr/ports/devel/gdb