No, GP heard some folsky etymology. Lǎowài does not mean ancient foreigner. One of 老 lǎo's meanings is an honorific like "old / venerable." 老伯 lǎobó uncle, 老闆 lǎobǎn boss, 老師 lǎoshī teacher, are all honorifics that don't mean really mean "old" (except for uncle, which you can use to address a man older than yourself, but doesn't mean he's from ancient times). 老人 lǎorén does mean "old people" but just regular old, not ancient old. 老外 lǎowài just means outsider.

古 gǔ means ancient, like 古人 gǔrén is ancient people. You could construct phrases to mean ancient foreigners, modern immigrants, modern foreign people, etc. but I don't know of any that would carry more special meaning than equivalent phrases in English.