Change an image in Gimp with a gradient like a shiny button or sticker tab

I have this image with a gradient as below:

I need to change it to the color #D21108 (type of RED). Of course, I do not want to paint it to this color, but to change the color to #D21108 and its shades (like I do in Photoshop via Brush > Colour Replacement). I have all info about this color, values of RGB, CMYK, Lab and HSV if you need them.

Now, I guess I can easily change it via GIMP's menu "Colors". I tried options Color Balance, Hue-Saturation, Colorize, but I did not find a way to enter the starting color I need. I was closest to my aim via Hue-Saturation, but I could not get this red value, no matter what I tried.

As I am not a graphics expert, please tell me how to accomplish this task.

PS. What's the meaning of Lab and HSV in the color scheme? I know the meaning of RGB and CMYK.

Answer

Brush > Colour Replacement

http://www.youtube.com/watch?v=Zm7BCw_E9wU

EDIT:
For GIMP -

Look at image title - if not RGB save it as .png or .gif
To change color wherever it is in entire image:
Select 'Color Tool' – (rectangular 3-color bar on top row)
Click on color to be changed
Select Edit and choose: Clear to eliminate color
Edit and fill with a) foreground color or b) background color