Sol-Gel Low Tg
I am effraid that sol-gel processing cannot changes drastically the Tg of the same theoretical composition as the one prepared by glass melting. The only thing which can make a difference is the amount of water incorporated in your glass by sol-gel.
Most of the low Tg glasses are phosphates based. Compositions in those systems can have extremely low Tg as low as 80°C (Tin-lead fluorophosphates)
Some old references:
Oxide glasses of very low softening point
N.H.Ray, R.J. Plaisted & W.D. Robinson
Glass Technology Vol 17 N° 2, 1976, 66
Id. Glass Technology vol. 14 N° 2, 1973, 50
There is no reason why the optical properties could be different from the melted glass.
It depends in what form the material should be prepared
For films in principle any geometry can be worked out.
For bulk samples that's another story. Drying to a monolithic form and consolidate to glass is not straightforward.