When factory boats are built in molds, especially the liners, workers tend to do a poor job of getting the glass matt fully pressed into areas where there is a hard 90 degree angle like that. This results in voids behind the gelcoat and makes it 'easy' to break, can't emphasize enough how common this is. I agree with the above assumptions that this most likely caused by dropping either a battery or the motor during a the re-power. Actually, I wouldn't even use the word 'drop', I bet if either heavy object were just 'set down' a little too fast could have caused that.
Do yourself a favor and buy a quality DeWalt angle grinder, its about $90. A good grinder has about 1000 useful applications besides just boat work. Mine is one of my most prized and indispensable tools, but then again I am a welder...
Also as suggested, grind out the damage and feather back maybe two inches on either side. Don't be shy with the epoxy and some fiberglass cloth tape, build it up nicely. DO NOT build it all the way level with the gelcoat, build it up level with the glass but below the gelcoat layer. Then do yourself another favor and go to the Spectrum Color website. You can purchase a 2 oz gelcoat paste kit that is color matched to your boat for $25. The thickened gelcoat is easy to apply and very forgiving to work with. Even if the area is going to be covered by the sole, this gives you a chance to advance that boat repair education you were talking about. You'll thank me later when you see how easy it is to do a spotless gelcoat repair.