Do you mean you "bought a
3x5 flag to replace my too big 4x6."?
I think I'd go with the blue location on a 10 foot pole, the top the gable should add another 10 ft. or more (I wouldn't know this). You'll need the extra height to get the wind to make even a 3x5 to fly well when there are houses and trees in close proximity. This is confirmed
here. My 3x5 is on a 20 ft. pole but it's real close to an open field and with only 1 of 4 sides restricted a bit (the house) so it flies very nice. And it's very well lit all night.
You probably know this but I'll say it anyway....a free standing J-pole by itself might not hold it well but if the J-pole was under the gable vent and the flag pole secured to the fascia board at the edge of the roof line it would add strength and stability. Or anything to add lateral strength to the vertical pole that the J-pole is expected to hold.
I've also used a 2x4 and 1x4 lumber in place of the metal horizontal shown on this antenna gable mount at the bottom. I'm not suggesting you go buy and use this, just showing to illustrate how the vertical mount could be strengthened with horizontally-mounted, treated or painted lumber and no $40 - $50 antenna mount needed.
Props to you for your patriotism and concern....
P.S.- Please don't fall off the roof or ladder!