Both a creature and a land must be targeted when the ability triggers. If there aren’t enough legal targets, the ability doesn’t go on the stack and Goblin Grenadiers can’t be sacrificed this way.

A permanent that’s both a land and a creature may be chosen as both of this ability’s targets.

An ability that triggers when something “attacks and isn’t blocked” triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.