One option nobody mentioned: Compose has a top-level include: (Compose v2.20+). In stack 2 add

include:
  - ../stack1/docker-compose.yml

and gluetun becomes part of the same project, so depends_on: gluetun: condition: service_healthy works again. The catch is that docker compose up/down in stack 2 now manages gluetun too, so it's best if stack 1 is only gluetun and everything else lives in its own files.