But in other checkers, all jumps are finished before the promotion happens!

E.g. official draughts rules:
"4.15. Confirming article 3.5., a man who passes during a multiple capture, over a square of his opponent’s promotion row, but ends on another row, remains a man at the end of the capture. "

Or brainking's rules for Draughts:
"However, if a pawn reaches the last row while jumping and can jump more pieces backwards, it does not promote and continues jumping as a pawn."

Turkish, Draughts (International), Canadian, Brazilian, Dameo all work that way - a piece jumps as much as it can and then gets promoted. (While English, Czech, Italian, Spanish / Portuguese, Thai doesn't allow backwards or side jumps, so a piece can never jump further once it reaches the king's row).

Only russian is an exception.

If Gothic is an exception too, I think it needs to be stated more clearly in the rules.

It is very weird to me as a multi-variant checkers players, to see a jump sequence aborted in the middle.