Board size changes the whole character of the game
In tic tac toe the board is small, and almost every cell becomes important immediately. In gomoku the board is wider, so games unfold more gradually. You have time to build long ideas, spread your attention across several areas and only later move to a direct resolution.
Because of that, tic tac toe is better for short series and quick decisions, while gomoku feels much more like a game about accumulating pressure and controlling space over distance.
The length of the winning line changes the rhythm
On a classic tic tac toe board you need a short line, which makes the price of every move very high straight away. In gomoku you have to build a longer combination, and that gives more time for preparation, decoy ideas and multi-move traps. The game becomes less instant but more layered.
So someone used to tic tac toe may at first be surprised by how long a gomoku position looks "unfinished". Yet a different kind of depth is born in exactly that unfinishedness.
Which is easier to get into
Tic tac toe is almost always simpler as a first game. It explains faster, plays faster and shows the cost of mistakes faster. Gomoku suits anyone who already wants more space and more time to develop a position. In that sense tic tac toe makes an excellent starting point before the larger line games.
If the goal is quick enjoyment from a short logical duel, tic tac toe usually wins. If you want a longer, unfolding game, gomoku feels richer.
Why the comparison helps you understand the game itself
Comparing tic tac toe with gomoku makes it easier to see that the strength of the first game lies not in scale but in clarity. It concentrates strategy so densely that almost every mistake shows up in the result immediately. That is what makes it especially good for learning and practice.
To widen the picture, it is worth looking at the comparison with Connect Four and the general piece on the history of the game.
Back to the compact version of the game
After the comparison it is especially easy to feel why fast browser games of tic tac toe are still so convenient.