VoLGA (Voice over LTE via Generic Access) is, or rather was, an approach to use GSM and WCDMA circuit-switched services over LTE. Depending on the speaker, VoLGA was proposed as an in interim solution before IMS VoLTE, or as an alternative to IMS VoLTE. Since then industry, and most importantly operators, have clearly indicated their support for IMS VoLTE, as defined by GSMA.
Perhaps the main drawback in the VoLGA approach was to tie LTE/EPC into GSM/WCDMA circuit core for years to come. This would have been strategically difficult choice, considering that LTE/EPC was specifically designed to be PS-only, allowing both voice and data to be served over packet core.