Make sure you have antialising on when you create your text. Because Gimp works with pixels your text will have a stair effect. There is a couple ways to minimise the effect.
Before adding your gradient, select your text and turn the selection to a path then back to a selection to add your gradient.
You can also try to blur your text a little. To keep it reasonably sharp select the text then add a 1-2 pixel border and only blur that
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