I don't think it's that simple. Balmung starts as a beam but spreads into aoe but still centers around Siegfried. But looking at how Galatine works in Extella animation it is literally Gawain slashing/swinging a super long blade made of light that is 13km length. Meaning it does not have to make contact with the beam to get to Siegfried because Gawain can swing it from the left, the right or even smash down from above like Musashi's NP.
That aside, the max range is notably shorter than Siegfried, so if Siegfried fires Balmung at maximum range (50) he is safe from a retaliation from max range Galatine (40). Even better, Lancelot managed to fight Gawain when he gets to close range because Galatine has a min range of 20, making Gawain unable to shinmei it on him. So if Siegfried can keep up with Gawain in melee and not let Gawain slash him (i.e blocking the blows or dodge) I'm sure he can stall Gawain till buff gone.
It will do more damage than Clarent Blood Arthur if it directly hits Siegfried. Remember, this is sunbuff Gawain so Galatine is also buffed. Of course we don't know if the boost is also x3 which is ridiculous boost so there's that.Of course yeah your right on the fact Galatine would be supported by the Sun-buff so it should overwhelm Balmung, tho i'm not sure whether it would be able to actually kill Siegfried or would do as much as Clarent Blood Arthur. Honestly who would win the duel i'd think would depend on how much damage Gawain does in his normal attacks as it gets past his armor.