Learn Dutch: Het was echt een gezellig praatje, hè? - It was a really nice chat, wasn't it?

The tag hè is perfect for casually asking someone to agree with a shared feeling or experience, just like saying 'wasn't it?' or 'right?'. You could use toch at the very end as well, but hè sounds much softer and more conversational for a friendly comment about a nice chat.

Could I use of niet at the end of this sentence instead?

You could, but it would change the mood. The tag of niet literally means 'or not' and sounds a bit more direct or challenging, as if you genuinely doubt they enjoyed it. Because gezellig implies a warm, friendly atmosphere, the softer hè is a much better fit.

Why doesn't the adjective get an -e at the end to make it een gezellige praatje?

This is because all diminutive words (words ending in -je, like praatje) are het words. A very important grammar rule to remember is that when a het word follows the indefinite article een, the adjective before it does not get an -e.

Why use the diminutive praatje instead of a regular noun like gesprek?