Goofs

The swimming pool at Gloria's house has water that is much too clean for a house that has been empty for a long time.

When Gloria and Oscar hears the alarm signal from the TV it is dark outside despite the fact that the alarm sounds just before the colossal appearing which is 8.05 am.

When Gloria comes home and frightened by Oscar the room is totally dark although the giant television is turned on so she should see Oscar in the armchair.

Although anything is possible, it's hard to believe that at the time that Gloria is in action at Seoul, people in the U.S.A. are watching a live broadcast, while enjoying a beer. At 8:05 AM.

Why is that hard to believe? Any time a bar is open, someone will come to drink. Night shift workers will for sure.

In the final flashback to Gloria and Oscar's youth, a comic book is shown falling from Oscar's backpack. The back cover of the comic features a partial quote from David S Goyer and mentions his films Batman Begins (2005) and The Invisible (2007). The flashback occurs twenty-five years before the present day of the film, so either Colossal is set in 2032 at the earliest -- which does not appear to be the case -- or the book is an anachronism.

The chair & computer and an active FaceTime call disappear when Oscar rings doorbell and brings Gloria a Futon Couch.

Gloria is depicted as being flat broke throughout the film but somehow manages to book a flight to distant Seoul and arrange for transportation from the airport to downtown Seoul and (presumably) accommodations while she is there.

If no one on the playground can see the monsters or anything else in Seoul, then Gloria in Seoul should not be able to see Oscar on the playground. And indeed she doesn't, but still manages to know where he is and pick him up.

If Gloria has been absent from town for many years, who has been maintaining her house and paying the electric bills?