Yes, if you apply the blur as a Smart Filter (by converting your layer to a Smart Object first), you can double-click the filter to reopen and adjust the blur settings at any time.
You can apply Gaussian blur by using the Select Subject or Select and Mask tools to isolate your subject, then invert the selection and apply Gaussian Blur to the background. This is a common technique for creating a
shallow depth-of-field effect.
Lightroom does not have a dedicated Gaussian Blur filter like Photoshop, but you can achieve a similar effect using the Radial Gradient, Linear Gradient, or Brush tools with the Clarity, Texture, and Sharpness sliders set to negative values. For more precise blur adjustments, Photoshop offers additional flexibility.