- Play Combustible Gearhulk even if the opponent can't be targeted (e.g. through Sigarda, Heron's Grace), a 6/6 first strike creature is decent.

forge-ai/src/main/java/forge/ai/ability/ChooseGenericEffectAi.java

@@ -72,6 +72,7 @@ public class ChooseGenericEffectAi extends SpellAbilityAi {
                     return true;
                 }
             }
+            return true; // perhaps the opponent(s) had Sigarda, Heron's Grace or another effect in play, still play the creature as 6/6
         }
 
         return super.doTriggerAINoCost(aiPlayer, sa, mandatory);