Here's my take...
The GG 250 is a great around bike and should not be overlooked. The 300 would be better if you like to short shift and lug around more. It also helps if you're in the wrong gear by accident and need the extra torque to pull you through mud or sand.
The suspension on GG is better out-of-box, but the KTM's can be just a good if reworked by a good tuner. My 200EXC has front and rear reworked by MX-Tech. It is very nice with no complaints... just as good as my old XC250. I rode my friends 250EXC and it was very harsh and rode like crap. Suspension should not be the deal-breaker on a bike purchase... it can easily be reworked.
KTM's are slightly easier to work on in my opinion. All bolts have common heads and they typically thread into threaded inserts. This lets you unbolt things quickly with a single wrench. Whereas, the GG makes you juggle two wrenches (such as removing the silencer). Not a big deal, but a slightly easier maintenance task.
The KTM has large aftermarket support with multiple brands to choose from. The GG has LTR for most key items and Flatland, Eline, etc.. for some of the other guards. Also, some parts crossover onto GG's, so you should be easily covered with any aftermarket item... just slightly fewer options available.
Parts available is excellent on either brand. I order everything by phone or online. A local dealer is not needed... the internet makes anywhere in the world your local dealer.
Good luck!