Misread the definition of the type_of_loader field; it should include a

version.

src/arch/i386/image/bzimage.c

 	struct bzimage_cmdline cmdline;
 
 	/* Update the header and copy it into the loaded kernel */
-	bzhdr->type_of_loader = BZI_LOADER_TYPE_ETHERBOOT;
+	bzhdr->type_of_loader = BZI_LOADER_TYPE_GPXE;
 	if ( bzhdr->version >= 0x0201 ) {
 		bzhdr->heap_end_ptr = ( load_ctx->rm_heap - 0x200 );
 		bzhdr->loadflags |= BZI_CAN_USE_HEAP;

src/arch/i386/include/bzimage.h

 #define BZI_SIGNATURE 0x53726448
 
 /** bzImage boot loader identifier for Etherboot */
-#define BZI_LOADER_TYPE_ETHERBOOT 4
+#define BZI_LOADER_TYPE_ETHERBOOT 0x40
+
+/** bzImage boot loader identifier for gPXE
+ *
+ * We advertise ourselves as Etherboot version 6.
+ */
+#define BZI_LOADER_TYPE_GPXE ( BZI_LOADER_TYPE_ETHERBOOT | 0x06 )
 
 /** bzImage "load high" flag */
 #define BZI_LOAD_HIGH 0x01