Can oxytocin travel from one cell to another via gap junctions?
Synaptic vesicles are on the order of tens of nanometers (for example Zhang et al give a mean diameter around 40 nm).
Gap junctions are much smaller, on the order of 1-2 nm. Maeda et al measure one at 1.4 nm.
Peptides the size of oxytocin present in the cytosol could theoretically travel through gap junctions. Vesicle-packaged peptides or molecules of any size cannot.

Maeda, S., Nakagawa, S., Suga, M., Yamashita, E., Oshima, A., Fujiyoshi, Y., &amp; Tsukihara, T. (2009). Structure of the connexin 26 gap junction channel at 3.5 Å resolution. Nature, 458(7238), 597-602.
Zhang, B., Koh, Y. H., Beckstead, R. B., Budnik, V., Ganetzky, B., &amp; Bellen, H. J. (1998). Synaptic vesicle size and number are regulated by a clathrin adaptor protein required for endocytosis. neuron, 21(6), 1465-1475.