True that. There exists a lot of reincarnation stories without transmigration ('Become a star', one of my favorite novels, is like that. It's a guy who has over 900 past lives since the beginning of times, and uses those in his acting career), there are also regression stories without transmigration (salaryman had an awful life, gets back in time and gets revenge, for exemple 'God of Money')
There is also transmigration through possession (another favorite of mine 'Death is the only ending for the villainess', where she suddenly enters the body of a villainess in a game), transmigration with reincarnation (most stories that we see here, really), and transmigration that is simple portal fantasy (like you mentionned, 'Gate')
In that case, which is isekai and which isn't ? Are only transmigration stories isekai ?