What are the “U” and “G” time formats?

The U and G are not WordPress specific. get_post_time() is using the same datetime formats as the default PHP date method.

Here is the documentation for all of the formats:
https://www.php.net/manual/en/function.date.php

Specific to the question:

U is Seconds since the Unix Epoch (January 1 1970 00:00:00 GMT)

G is 24-hour format of an hour without leading zeros. 0 through 24