Presumably, the vertical gradient is to what you refer?
This effect can be done in SFML simply with two very different methods.
The first would be to use a bitmap text instead of a scalable text (like TTF, for example).
One example of this is Selba Ward's
Bitmap Text:
The second would be to use a fragment shader. Fragment shaders adjust the pixel dependant on a multitude of things but included is the pixel position so you can, for example, just adjust the colour based on the y position of the fragment (pixel). Note, though, that OpenGL fragment shaders use non-inverted y axis; that is, the opposite to SFML - the y value increase as it goes up instead of down.
These can seem to be a bit complicated at first but once you get used to them, they are very powerful and flexible, and - I expect - would be the ideal approach for your situation.
EDIT:
In addition, there is another rather simple solution but adds additional overhead (cost of the simplicity):
Render the text to a render texture and the draw that render texture with a quad (vertex array). This vertex array can have different colours per vertex so each corner can have a different colour. This allows both the top corners to be the same and both the bottom ones to be the same. However, the top and bottom can be different, causing linear interpolation - vertically - causing a gradient. If you want to try this method, remember to render the text as while (on a transparent render texture) and note that you can only pick two colours (the top and bottom) so you cannot have it go from one colour to another and back again without splitting the quad into pieces (although that is relatively simple to do too!).
