Under free opening rules, Gomoku has been proved to be sure-win for black, aka the first player. This work done by Victor Allis could be part of the reason why Gomoku is undeservedly treated as a toy board game.
We have to remember, however, that this proof is computer search based. There was never an algorithm to find the best move given a random position. In other words, it only proves that these certain openings will guarantee black a win.
Those who loved the game came up with new opening rules. The world championships in 1989 and 1991 used the pro rule. After some evolutions, swap2 now became the standard. Under Gomoku swap2, the first player puts two black stones and one white stone on the board, and the second player can either pick one color, or puts two more stones and gives back the power to choose color to the first player.
I know about Connect6, and also a branch called Renju. But I find Gomoku more attractive. You should totally try it sometimes.
https://en.wikipedia.org/wiki/Connect6