GIMP 2.10

How tos

Select Layer → Transparency → Add Alpha Channel
Select the background using the Fuzzy Selection Tool (Magic Wand) and press the Del key.

GIF will allow you single-color transparency, while PNG has full alpha channel support.
See also GIMP Watermark removal

Select layer with color area to be changed, Select –> All, select this color area with the Fuzzy Select Tool, Select –> Invert.
Select layer with text and click text within color area, this should result in selection around text within color area. Make layer with color area invisible.
Click Select –> Selection Editor, set Opacity to 100, Threshold to 80 (normal font weight) or 120 (bold), Fill by to Alpha, and enable Antialiasing. Shrink the selection by 1px in Select –> Shrink.
Select Bucket Fill Tool, choose foreground color, and click on each letter within the color area.

Layer –> New Layer and select it
Make a selection, for example a circle with the Ellipse Select Tool by holding down CTRL-SHIFT while drawing
Select –> To Path
For bottom (footer) text omit next step
For top (header) text select the Flip Tool, Direction Horizontal, then click into the circle path of the same layer
Select the Text Tool, choose any location in the same layer then write and format the text
Right-click on the text or right-click on the text layer to the right, select Text along Path
Add another new layer and select it, then Select –> From Path
Select the Bucket Fill Tool and fill the selection with the foreground color
Select –> None
Select the Rotate Tool and rotate the text, then hit the Enter key on your keyboard
Layer –> Layer to Image size
Delete any layers not needed anymore

Select a path with the Fuzzy Select Tool, Ellipse Select Tool, etc.
Select menu Select → To Path
Select the Flip Tool
Select Affect: Path and Flip Type: Vertical
Click into the layer with the selection to flip the path
Select the Text Tool and type the text
Right click into the text box and select Text along Path
Select menu Layer → New Layer
Select Path to Selection in window Paths - Brushes / Open the paths dialog
Select the Pencil Tool and paint the text
Select menu Layer → Transform → Arbitrary Rotation, set the center point, and rotate the text
Right click on the temporary layer Floating Selection in window Layers - Brushes and select To new Layer
Delete any layers not needed anymore

Right-click on the layer, select Layers –> Add Alpha Channel
Right-click on the layer, select Layers –> Duplicate Layer (not necessary, just to be save to have a backup)
Make copy visible only, then Select All, then select the Fuzzy Select Tool and click into the background area
Select Selection to path, and again Path to selection, and make the path visible
Right-click on the layer, select Add Layer Mask…, choose Selection and click Add
Go to Image –> Fit Canvas to Selection, and save

Resynthesizer allows you to remove watermarks and similar from an image. Download for GIMP 2.10 from first link below (compiled 2020).

GIMP portable: Move all files (not folder) to GIMPPortable\Data\.gimp\plug-ins\
GIMP installed: Move all files (not folder) to C:\Users\user\AppData\Roaming\GIMP\2.10\plug-ins\