Summary: A derasha on a gershayim in Behaaloscha, that the doubling of the stroke implies twice, and that it is on a lamed makes for double 30. I disagree with the need, or inclination, to darshen it, and try to explain why a gershayim instead of a geresh. But to each his own. I just discuss this for the sake of completeness.
Post: Since I am posting on Birkas Avraham and darshening trup, I might as well take note of the other place in Behaalotcha that he darshens trup. This time, a gershayim.
"Then the Lord said to Moses, "Assemble for Me seventy men of the elders of Israel, whom you know to be the people's elders and officers, and you shall take them to the Tent of Meeting, and they shall stand there with you."
Birkas Avraham writes:
"The trup of gershayim on the letter lamed doubles the count, to twice thirty"...
"In the word לי there is a trup symbol of gershayim on the lamed, and it is possible to explain that it comes to double the lamed, and to hint to that which is stated in the midrash Esfa (which is brought in Yalkut Shimoni) that Moshe stood them on the three sides of the Mishkan, which altogether has a perimeter of 70 cubits, each man having one cubit -- and that is thirty on the north side, thirty on the south side, and ten on the west side, while Moshe stood upon them by the east side."
(See the details of this midrash here.)
My strong inclination is that trup in general is not to be darshened like this, for this was not the likely intent of the trup. What can account for the gershayim, on a 'peshat'-level?
Well, gershayim is basically a geresh, so it can come in the contexts where a geresh is expected. The doubling of the geresh stroke is a matter of orthography of trup, which came into being post-Talmudically. This presumably to demonstrate that it is more developed musically, but still related in value to the geresh. (This rather than being a message to double something in the text.) And indeed, in the superlinear Babylonian trup system, there is no orthographic distinction between geresh and gershayim, with both as the initial letter of טרס over the word.
I am not certain of the precise technical rules which dictate when a gershayim appears instead of a geresh. One salient feature is that gershayim always appears on milera words, and never on ones which are mile'el. This would suggest that the considerations are melodic and have to do with stress and syllable count, rather than with hints to midrashim.
Indeed, there is a makef (a horizontal dash) in אספה-לי, and a maarich (/gaaya) in the aleph. So the gershayim is on the whole word אספה-לי, and is placed specifically on the lamed because it is the last syllable of this long 'word'. What if we had divided it into אספה and לי? Well, the geresh is due on לי, and we could have munach on the previous word or a kadma. But we already have kadma ve'azla (azla = geresh) on Vayomer Hashem. And anyway, the makef joining אספה-לי seems to carry some semantic weight, such that the לי is almost null or else syntactically a morphological element, subservient to אספה; meanwhile, as a separate word, it would have brought more stress to the word לי. And once the words are joined together, with a certain number of syllables and with a stress on the last syllable (לי), perhaps this would trigger the gershayim, or increased likelihood of gershayim.
Again, most (though not all) of this is speculation. But I think that this is the more productive line of investigation, while imagining connections to midrashim is less productive.