The Gitrog Monster
Nostalgic Dreams

Seeing as Nostalgic Dreams is being reprinted in Vintage Masters.

If I discard multiple cards to pay the additional cost, including multiple land cards, are the cards discarded simultaneously as one happening or sequentially as multiple happenings?

Could the last ability on The Gitrog Monster trigger more than once during the payment?

The cards are discarded simultaneously; the additional cost of Nostalgic Dreams is one instance of “discard X cards”, not X separate instances of “discard a card”. As such, The Gitrog Monster can only trigger once during the casting of Nostalgic Dreams, no matter how many land cards are among those discarded.