Cannot be Summoned unless you control a face-up "Gravity Axe - Grarl". If this is the only card in your hand, you can Special Summon it (from your hand).
Rulings
Individual Card FAQs
If you wish to use "Guardian Grarl"'s effect to Special Summon him from your hand, you still must have "Gravity Axe - Grarl" on your side of the field.
You cannot Tribute Summon "Guardian Grarl" by Tributing a monster equipped with your only "Gravity Axe - Grarl" on your side of the field, because the "Gravity Axe - Grarl" will no longer be on the field when the Summon occurs.
EARTH|
Level 5|[ Dinosaur / Effect ]
ATK 2500|
DEF 1000