Ok, so release of 50 gigatons of CH4 means 10x increase in atmospheric CH4, then we are looking at around 50% contribution to greenhouse effect from CH4 after that I guess.

That's basically double of what CO2 contributes today, what will also increase the amount of water in the atmosphere.

But CH4 is short lived, so the most important question is how fast it will be emitted. (And the second may be: could we make it even shorter lived?)