What's the Japanese 3rd tier like?
Taken from Wikipedia
Rules for promotion to J2 are largely similar to those of Japan Football League in recent seasons: to be promoted, a club must hold or be granted a J2 license and finish in top 2 of the league. Since 2017, the champions and the runners-up have been promoted directly and replace the 21st- and 22nd-placed J2 clubs. If only the champion or runner-up holds or is given a J2 license, only the bottom club of J2 is relegated; if both top 2 finishers are ineligible for promotion, then no teams will be promoted to or relegated from J2. For promotion purposes, under-23 teams are excluded from the final table when considering which teams are eligible for promotion. An extreme example would be the following scenario: If all three under-23 teams finished in the top 3, then the fourth- and fifth-placed teams would be eligible for promotion (subject to the license requirement).[13]
No relegation to JFL is planned in the foreseeable future.
They've literally scraped by for several seasons, so imagine if they stayed up on this technicality! Not currently sure what the status is of the J3 1st and 2nd place is though. Although I'm assuming that there is no way out for Gifu as it currently stands.