> how does Gleam determine that the expression continues on the second line?

The fact that it isn't obvious means the syntax is bad. Stuff this basic shouldn't be ambiguous.

> Go's lexer inserts a semicolon after the following tokens if they appear just before a newline ... [non-trivial list] ... Simple enough!

Again I beg to differ. Fundamentally it's just really difficult to make a rule that is actually simple, and lets you write code that you'd expect to work.

I think the author's indentation idea is fairly reasonable, though I think indentation sensitivity is pretty error-prone.