I understand why the announcers are encouraged to refer to the yardage point that will give a first down as “the line to gain” since it is a line and not a “first down” in itself. But it’s awkward and doesn’t sound like a football term.
Which is why I suggest “line to go” to represent the first down line, since it’s in keeping with “first and goal-to-go” and so forth. So, if you’re a producer or an announcer reading this, try it. I suspect it will catch on in a way that “line to gain” won’t.