A study by Black & Veatch a while back showed that for intermediate and peaking operating profiles (< 5000 hours per year), gas-fired combustion engine plants beat any OCGT plant in lifetime feasibility. This is just not a well-known fact, as the "standard" approach for energy consultancies is to test feasibility on the assumption of 8000 hours per year base load operation (and why not, since all GTs behave rather similarly in cyclic operation, and combustion engines are usually not considered as an option).
Regarding pure base load plants, you are correct, modern CCGTs are the most efficient solution. However, looking at recent developments in some Western countries, it is uncertain how much of pure base load capacity will be required. For instance, Spanish utilities built vast amounts of CCGT capacity during the last decade, but due to the increase of wind and solar, the average capacity factor for these brand new, very efficient CCGTs now sits around 30%. In private discussions some utility people have confided that they're incurring heavy losses and only running the CCGTs because of take-or-pay gas contracts.
Regarding capacity, 1/4 of Wärtsilä's installed base is in power plants. And, regarding the common conception of "diesel engines": nowadays a majority of the engines Wärtsilä sells for power plants are gas-fired.