The GDC binary is named egdc on OpenBSD.

Index: source/dub/dub.d
--- source/dub/dub.d.orig
+++ source/dub/dub.d
@@ -1409,7 +1409,7 @@ class Dub {
 		version (Windows) enum sep = ";", exe = ".exe";
 		version (Posix) enum sep = ":", exe = "";
 
-		auto compilers = ["dmd", "gdc", "gdmd", "ldc2", "ldmd2"];
+		auto compilers = ["dmd", "egdc", "gdmd", "ldc2", "ldmd2"];
 		// If a compiler name is specified, look for it next to dub.
 		// Otherwise, look for any of the common compilers adjacent to dub.
 		if (m_defaultCompiler.length)