try something

.env

@@ -0,0 +1,2 @@
+NIM_COMMIT=dumpcells
+

beacon_chain/rpc/rest_nimbus_api.nim

@@ -428,3 +428,7 @@ proc installNimbusApiHandlers*(router: var RestRouter, node: BeaconNode) =
         all_peers: allPeers
       )
     )
+
+  router.api(MethodGet, "/nimbus/v1/debug/memory/gc_cells") do (
+    ) -> RestApiResponse:
+    return RestApiResponse.jsonResponse(GC_dumpCells())

env.sh

@@ -32,5 +32,7 @@ if [ -f "${USER_ENV_FILE}" ]; then
   set +o allexport
 fi
 
+
 source ${ABS_PATH}/vendor/nimbus-build-system/scripts/env.sh
 
+NIM_COMMIT="dumpcells"