We study structural and chemical transformations induced by focused laser beam in GaAs nanowires with axial zinc-blende/wurtzite (ZB/WZ) heterostucture. The experiments are performed using a combination of transmission electron microscopy, energy-dispersive X-ray spectroscopy, Raman scattering, and photoluminescence spectroscopy. For the both components of heterostructure, laser irradiation under atmospheric air is found to produce a double surface layer which is composed of crystalline arsenic and of amorphous GaO$_{x}$. The latter compound is responsible for appearance of a peak at 1.76 eV in photoluminescence spectra of GaAs nanowires. Under increased laser power density, due to sample heating, evaporation of the surface crystalline arsenic and formation of $β$-Ga$_{2}$O$_{3}$ nanocrystals proceed on surface of the zinc-blende part of nanowire. The formed nanocrystals reveal a photoluminescence band in visible range of 1.7-2.4 eV. At the same power density for wurtzite part of the nanowire, total amorphization with formation of $β$-Ga$_{2}$O$_{3}$ nanocrystals occurs. Observed transformation of WZ-GaAs to $β$-Ga$_{2}$O$_{3}$ nanocrystals presents an available way for creation of axial and radial heterostuctures ZB-GaAs/$β$-Ga$_{2}$O$_{3}$ for optoelectronic and photonic applications. ; Comment: 12 pages, 5 figures