EDIT: and BIT is the acronym of the officilal name of the fund (Bitcoin Investment Trust), while GBTC is the ticker symbol assigned by OTCQX for the BIT shares. There are no "GBTC shares".
GBTC are shares of BIT that have been deposited for sale on OTC. Therefore, not every share of BIT is represented by a share of GBTC. It's not only an acronym. GBTC is a share of BIT that is for sale.