“La une” is the headline in English. So the idiom “Faire la une” is simply “Make the headlines” in English.

It’s not a matters of gender of “Journal”, since in this context “une” is used as a noun and not an adjective. “La une” is an idiom, It refers to the fact that important news is on the front page on newspapers, on the first page, the one, “la une”.

That’s just it.

“La une” means “the front page” or “the headlines”.

It’s simply short for “la page une” (page one), that’s why it’s feminine and we kept la instead of l’.

I also found interesting information in the answer for the possible duplicate, especially why “le un” and “l’un” can both be used, but for different things. Essentially we say “le un” when referring to the number, as opposed to the article.

Moreover it is very common when numbers are associated with things (like pages, channels, or in a contest where contestant have numbers, etc.) to refer to those things directly by the number.

You’re also note that “channel one” is “la une“, for the same reason.