Granted, even if Methane was 100% efficiently produced your still down to 60% from burning it. However, the main advantage of Gas turbines is they are cheap when not in use. Paying even 1$/kWh 1% of the time to provide an extra 10% power in those rare cases is really not a big deal. (1$ * 0.01 * .1 adds 1/10th of a cent per kWh to average electricity prices.)
Sure, for basic day night cycle adjustments your going to want hydro or battery storage etc, but you start needing a lot of storage that's very rarely used if you want 99.99+% up-time.
PS: This of course assumes a significant oversupply on average.