I am a member of an SCI subject to income tax (tax-transparent). The authorised manager is selling one of the SCI’s properties. Will it be subject to real estate capital gains tax?

Yes. When an SCI is subject to income tax, the rules governing the taxation of capital gains are the same as those applicable to individuals (rates, allowances based on the length of ownership, etc.) (Art. 150 U of the French General Tax Code and BOI-RFPI-PVI-10-10).