GTPase tethering membranes through formation of trans-homooligomers and mediating homotypic fusion of endoplasmic reticulum membranes. Functions in endoplasmic reticulum tubular network biogenesis. Interacts with REEP5 and RTN3. Interacts with ZFYVE27. Localizes at endoplasmic reticulum (ER) three-way tubular junctions. Belongs to the TRAFAC class dynamin-like GTPase superfamily. GB1/RHD3 GTPase family. GB1 subfamily.