The picture looks fine at first sight:

A closer look (crop of left part) reveals a speck in the blue sky..  

No, it's not a cloud; it is not a speck on your screen. It's a dust spot on the camera sensor.

A picture of a white surface taken using a small aperture (F16)..

.. shows the dust spot (left): 

It is quite common that dust enters the interior of retractable lens cameras. As long as you are not shooting the sky or a plain bright surface, there might be no visible effect.  Particles that settled on the upper part of the sensor become more likely visible in scenery shots.

What are the options? 

• Sensor cleaning by qualified service center. Might be expensive if guarantee has expired.
• Do-it yourself following instructions provided by other users (example: instructions for GR sensor cleanings). Disadvantage: loss of guarantee when opening the camera.
• Shoot at F2.8 only
• Remove the spots in post-processing
• Vibration

As the guarantee of my GR had expired, vibration seemed a promising option. However, I wouldn't have tried the following if I hadn't acquired a second-hand backup. 

The dust infected camera is marked "SS" (sensor stain). A KitchenAid mixer is considered suitable to do the job. The GR SS is attached to the mixer using tape. 

The vibration of the mixer is transferred to the GR mechanically and acoustically (see video for the later).

After this treatment, the sensor is checked again (picture of white surface @F16, see above) and the procedure is repeated. Mixing a smoothie at the same time helps to overcome the frustration in case of failure.

Optimized eccentric fixation of camera on the mixer to increase the vibrations:

Not achieving the desired results, the below experimental set-up was considered but not realized..

Instead, and also because of being fed-up with smoothies, another approach using the coffee machine was tried:  

Grinding coffee beans produces a fair amount of vibrations although the acoustic treatment is not so impressive:

So far the dust spot is still firmly attached to the sensor. Any future success will be communicated here.