The bubble issue is related to the increased solution viscosity after mixing the gel solution with cell medium. Here are some suggestions that can help to reduce the formation of bubbles:
Warm up the VitroGel solution to 37° C to reduce the viscosity of the gel.
Gently mix the VitroGel solution with the cell medium. Then, pipette slowly without introducing bubbles.
Quickly spin the mixing tube to get rid of bubbles.