Hard to say exactly; there aren't any gas-fired CCS projects I'm aware of. But we can do some "rule of thumb" guesstimates.
Probably the best approach is to look at an IGCC (a coal gasification based generation technology) CCS plant. Probably the furthest advanced example is the Kemper County plant in the US.
An IGCC plant is basically made of three components: a coal gasification unit (which makes the coal into CH4), a gas reformer (which converts the CH4 into separate streams of H2 and CO2) and a CCGT gas turbine optimised to burn hydrogen. The H2 from the reformer goes to the turbine, the CO2 to sequestration.
Were you to use gas, obviously the gasification stage would be redundant. The reforming stage would still be required.
Kemper's costing about $5.5Bn for a 580MW plant which can capture about 65% of the carbon content of the coal. In the US a CCGT unit without CCS would typically come in at about $800-1Bn/GW.
So, crudely scaling Kemper to 1GW gives us $9.5Bn/GW, of which something like $1.2bn will be for the adapted CCGT.
That gives us $8.3Bn for the "balance of plant". If we crudely assume that's split half and half between the reformer and the gasifier, and that as this is "FOAK" (First Of A Kind) plant that costs double what a series build unit would cost. we get about $2.1Bn for a series built reformer capable of supplying a 1GW unit.
That has the whole system coming in at about $3.1Bn - 3 to 3 1/2 times the cost of a standard CCGT. for a comparison, that's not dissimilar to the cost of an AP1000 PWR unit.
The system also absorbs some of the output of the plant - we'll assume about 20% for a gas fired CCS CCGT. That equates to a 25% increase in fuel usage.
The underlying cost of power from a CCGT is usually about 80% fuel and 20% fixed (operations, capital and finance). In the UK gas can currently produce at breakeven at about £60/MWh.
So, gas cost is about £48/MWh at the moment, and other costs about £12.
On the basis of the above, you'd expect gas costs per MWh of output to rise to about £60/MWh, and other costs to rise to about £42/MWh - giving a total of £102/MWh.
Which is somewhat more than the Hinkley C strike price, or that for new onshore wind. But it's predicated on a few key assumptions/omissions
1 - that the technology can be matured to a stage where series build cost reductions can be reached.
2 - that gas prices stay where they are.
3 - that there's no carbon cost - I'd expect a system like the above to outperform Kemper in terms of carbon capture, but not to approach 100% - I'd guess in the 80-90% range, giving CO2 output of 45-90g/KWh. It'd be a small contribution, though.
4 - I've not the faintest idea of the costs of the sequestration system - compression, piping and disposal in geological formations. I'd be staggered if it came in under £5/MWh, surprised at £10, though.
So, not easy to develop, and certainly not cheap!