I am trying to compare two strings in an if statement, but it is giving me an error. Why?

itemsInExistence = []
#items are put into this matrix in a save function
#Ex. ['Sword Mk II', 4, 4]
gains = random.randint(1, 3)
if gains == 2:
  gained_weapon = random.choice(itemsInExistence)
  print("You gained the item", gained_weapon)
  itemMatrix.append(gained_weapon)
  for i in itemsInExistence:
    if gained_weapon == itemsInExistence[i]:
      del itemsInExistence[i]
      break

It keeps giving me the error:

if gained_weapon == itemsInExistence[i]:

TypeError: list indices must be integers or slices, not str.

How would I go about fixing this without seriously changing how the items are set up?