This is caused by an incorrect suffix on your group name. The prefix is:
And the suffix is:
The font tag is deprecated and the strong tag is not closed properly. You should change the prefix to:
And change the suffix to:
I experimented with a feature like this with my themes. I only wanted a setting to be displayed if a previous setting was set to a certain value.
What I later realised is that the conditional setting will only be displayed once the existing setting has actually been saved. There isn't any ajax functionality which would make the "Gradient direction" setting appear as soon as "Radial gradient" is selected, which actually results in a pretty poor experience for admins since they won't see the new setting until they've saved the existing page and reloaded it.
In my own themes, I've simply added a description to situations like this which basically says "If Radial Gradient is selected above, this setting will control its direction." I feel like this may also be the best solution in your case.