Use a Mousse Instead of a Gel
Gel is not the best choice for thinning hair. Gel binds hair together, making your scalp more visible. Instead use a mousse. It dries softer and can add volume.
My favorite is AG Foam Weightless Volumizer. Along with this, using duckbill clips to add volume at the crown of your head helps. The last step in this process is to dry your curls with a diffuser while hanging your head upside down. This really gets the volume pumped up.