I found it very difficult. I'm using GIMP but can switch to any Linux/chrome/online graphics software.
If you need to keep the same "thickness" but want to increase the span, you can:
- Create a text layer with a curly bracket
- Layer>Text to path
- With the path editor:
- Isolate the tips by removing a segment between two anchors in the vertical parts
- Move the tips
- Reconnect the tips
- Select>From path and bucket-fill selection
Result: (top is original bracket from "URW Palladio L Bold")
But Gimp isn't really meant for this kind of text processing, look into a typesetting application such as Scribus.
I just downloaded SVG from https://thenounproject.com/ - they have a decent collection of brackets, arrows and all kinds of other icons. Then imported my image to Inkscape, add brackets and export.