Ah, I missed the gist of your question, which is indeed a bit more difficult. Rumil's answer is of course possible but it requires some slightly fancy construal and certainly Occam would prefer just one trip across the mountains.
A possibility, at least logically, is that different mountains are being referred to in the two quotes. But the RGEO quote specifically refers to the Ered Luin, so that option isn't promising. (Forgetting that the mountain range was specified in the RGEO quote, I speculated that she crossed the Blue Mountains before the fall of Gondolin and later crossed the Misty Mountains after the fall of Thangorodrim, but alas, that doesn't work.)
I suppose one could always fall back upon the explanation that Galadriel simply mis-spoke in LotR, or mis-remembered the details of her movements - her statement was, after all, made thousands of years after the events she's recalling. On the other hand, I'd expect that if one had been in Beleriand between the fall of Gondolin and the Great Battle, one would darn well remember it.
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