Ubiquitin-like modifier that increases cell-surface expression of kappa-type opioid receptor through facilitating anterograde intracellular trafficking of the receptor. Involved in formation of autophagosomal vacuoles. While LC3s are involved in elongation of the phagophore membrane, the GABARAP/GATE-16 subfamily is essential for a later stage in autophagosome maturation. The precursor molecule is cleaved by ATG4 (atg4a, atg4b, atg4c or atg4d) to expose the glycine at the C-terminus and form the cytosolic form, gabarapl1-I. The processed form is then activated by apg7l/atg7, transferred to atg3 and conjugated to phosphatidylethanolamine (PE) phospholipid to form the membrane-bound form, gabarapl1-II. During non-canonical autophagy, the processed form is conjugated to phosphatidylserine (PS) phospholipid. Atg4 proteins also mediate the delipidation of PE-conjugated forms required for gabarapl1 recycling when autophagosomes fuse with lysosomes. In addition, some atg4 proteins mediate delipidation of ATG8 proteins conjugated to PS during non-canonical autophagy. Belongs to the ATG8 family.