Coombs I just don't see how Arsenal have the responsibility to prepare players for other teams who aren't even playing competitive games.
Already said it, but Arsenal is accountable for what Arsenal can control. In this case whether Gabriel, who was nailed on to get a call-up from Brazil, had played six 90s in 20 days before he joined the Brazil camp.
We can only conjecture whether that influenced the chance of Gabi picking up a groin strain, but since he was training separately, it seems likely Arsenal knew he was knackered so Arsenal or Gabi asked Brazil to put him on lighter duties. Not playing him so much was an alternative Arsenal could have chosen.
It really is only conjecture. A groin strain can be provoked by overuse, sudden extensions, or by a return to high intensity activity following a rest.