It is built but not installed.
https://bugs.gentoo.org/show_bug.cgi?id=459630

Index: gpac/Makefile
===================================================================
--- gpac.orig/Makefile
+++ gpac/Makefile
@@ -59,6 +59,9 @@ install:
 	$(INSTALL) -d "$(DESTDIR)$(prefix)/bin"
 ifeq ($(DISABLE_ISOFF), no)
 	$(INSTALL) $(INSTFLAGS) -m 755 bin/gcc/MP4Box "$(DESTDIR)$(prefix)/bin"
+ifeq ($(DISABLE_M2TS), no)
+	$(INSTALL) $(INSTFLAGS) -m 755 bin/gcc/mp42ts "$(DESTDIR)$(prefix)/bin"
+endif
 endif
 ifeq ($(DISABLE_PLAYER), no)
 	$(INSTALL) $(INSTFLAGS) -m 755 bin/gcc/MP4Client "$(DESTDIR)$(prefix)/bin"