According to the examples given on the page about demonstrative pronouns on the Span¡shd!ct website, the rule is given that "they all must [match] the nouns they replace in gender and number". This is also clear from the examples provided on that page. Most of examples are pretty straightforward, where the demonstrative pronouns are used as a direct replacement for a missing noun.
However, there is this example: "Aquel es el Gran Cañón".
In the Spanish course provided by Babbel, something called "neutral demonstrative pronouns" is taught. It is behind a paywall, but if you have a subscription, navigate to this lesson: Grammar » Nouns and pronouns » The neutral demonstrative pronouns.
According to Babbel, when a demonstrative pronoun is used in this contex, it does not match the gender (there is no plural examples), so we have:
Aquello es un pájaro. (not "Aquel")
Aquello es una playa. (Not "Aquella")
Eso es una planta. (Not "Esa")
Below is a screenshot of this page in Babbel:
So which form is correct:
Aquel es el Gran Cañón.
Aquello es el Gran Cañón