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.