You can't assess a drop rate or rarity of any exotic except the special ones like Vex, Elder Ciphers and Necrochasm. All the other generic ones are pure luck and randomness. They don't vary from person to person either. It's just dumb luck. Gjallarhorn has the same drop rate as any exotic, from the patience and time to the hardlight.
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While that makes a lot of sense, I believe the Gjallarhorn has a much lower drop rate, as I have seen repeats of every other exotic in the game, and not one Gjallarhorn, even at 1500 hours into the game. That cannot be simply a coincidence as guns such as the last word and icebreaker are extremely common, and I do not see anyone complaining about not having those guns, while the Gjallarhorn is many veteran players last exotic to strive for.