/-- Define `GCDMonoid` on a structure just from the `lcm` and its properties. -/
noncomputable def gcdMonoidOfLCM [DecidableEq α] (lcm : α → α → α)
    (dvd_lcm_left : ∀ a b, a ∣ lcm a b) (dvd_lcm_right : ∀ a b, b ∣ lcm a b)
    (lcm_dvd : ∀ {a b c}, c ∣ a → b ∣ a → lcm c b ∣ a) : GCDMonoid α :=
  let exists_gcd a b := lcm_dvd (Dvd.intro b rfl) (Dvd.intro_left a rfl)
  { lcm
    gcd := fun a b => if a = 0 then b else if b = 0 then a else Classical.choose (exists_gcd a b)
    gcd_mul_lcm := fun a b => by
      -- Porting note: need `dsimp only`
      dsimp only
      split_ifs with h h_1
      · rw [h, eq_zero_of_zero_dvd (dvd_lcm_left _ _), mul_zero, zero_mul]
      · rw [h_1, eq_zero_of_zero_dvd (dvd_lcm_right _ _), mul_zero]
      rw [mul_comm, ← Classical.choose_spec (exists_gcd a b)]
    lcm_zero_left := fun a => eq_zero_of_zero_dvd (dvd_lcm_left _ _)
    lcm_zero_right := fun a => eq_zero_of_zero_dvd (dvd_lcm_right _ _)
    gcd_dvd_left := fun a b => by
      -- Porting note: need `dsimp only`
      dsimp only
      split_ifs with h h_1
      · rw [h]
        apply dvd_zero
      · exact dvd_rfl
      have h0 : lcm a b ≠ 0 := by
        intro con
        have h := lcm_dvd (Dvd.intro b rfl) (Dvd.intro_left a rfl)
        rw [con, zero_dvd_iff, mul_eq_zero] at h
        cases h
        · exact absurd ‹a = 0› h
        · exact absurd ‹b = 0› h_1
      rw [← mul_dvd_mul_iff_left h0, ← Classical.choose_spec (exists_gcd a b), mul_comm,
        mul_dvd_mul_iff_right h]
      apply dvd_lcm_right
    gcd_dvd_right := fun a b => by
      -- Porting note: need `dsimp only`
      dsimp only
      split_ifs with h h_1
      · exact dvd_rfl
      · rw [h_1]
        apply dvd_zero
      have h0 : lcm a b ≠ 0 := by
        intro con
        have h := lcm_dvd (Dvd.intro b rfl) (Dvd.intro_left a rfl)
        rw [con, zero_dvd_iff, mul_eq_zero] at h
        cases h
        · exact absurd ‹a = 0› h
        · exact absurd ‹b = 0› h_1
      rw [← mul_dvd_mul_iff_left h0, ← Classical.choose_spec (exists_gcd a b),
        mul_dvd_mul_iff_right h_1]
      apply dvd_lcm_left
    dvd_gcd := fun {a b c} ac ab => by
      -- Porting note: need `dsimp only`
      dsimp only
      split_ifs with h h_1
      · exact ab
      · exact ac
      have h0 : lcm c b ≠ 0 := by
        intro con
        have h := lcm_dvd (Dvd.intro b rfl) (Dvd.intro_left c rfl)
        rw [con, zero_dvd_iff, mul_eq_zero] at h
        cases h
        · exact absurd ‹c = 0› h
        · exact absurd ‹b = 0› h_1
      rw [← mul_dvd_mul_iff_left h0, ← Classical.choose_spec (exists_gcd c b)]
      rcases ab with ⟨d, rfl⟩
      rw [mul_eq_zero] at ‹a * d ≠ 0›
      push_neg at h_1
      

Proof state:
case neg.intro
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : DecidableEq α
lcm : α → α → α
dvd_lcm_left : ∀ (a b : α), a ∣ lcm a b
dvd_lcm_right : ∀ (a b : α), b ∣ lcm a b
lcm_dvd : ∀ {a b c : α}, c ∣ a → b ∣ a → lcm c b ∣ a
exists_gcd : ∀ (a b : α), lcm a b ∣ a * b := fun a b => lcm_dvd (Dvd.intro b rfl) (Dvd.intro_left a rfl)
a c : α
ac : a ∣ c
h : ¬c = 0
d : α
h0 : lcm c (a * d) ≠ 0
h_1 : a ≠ 0 ∧ d ≠ 0
⊢ lcm c (a * d) * a ∣ c * (a * d)

Next tactic:
rw [mul_comm a, ← mul_assoc, mul_dvd_mul_iff_right h_1.1]