The inbuilt Colorize Tool from Gimp is hardcoded to only allow only colorization by hue, saturation, and lightness. There is no colorize tool which allows setting color exactly by RGB values.
Still, it is very easy to colorize a greyscale image using the Bucket Fill tool where we can define any foreground color not only by HSV but also using RGB, a HTML color noration, the colour wheel, or the color picker tool.
In addition, we are able to adjust the opacity of the colorize effect, and we do have several fill mode options:
Bucket fill mode Addition (foreground fill to RGB 255,0,0, opacitiy 100):
Bucket fill mode Color (foreground fill to RGB 255,0,0, opacity 100):
Because bucket fill is so easy to configure and use I never missed another colorize tool.
Depending on your needs you may also find a plugin to add more sophisticated colorize tools (e.g. colorizing with more than one color in the G'Mic tools)