Gesha i known for its citric/acid flavour. And in this case I will recommend to extract your espresso with a shorter time with a course setting.

It appears very thin at the end of extraction but thats quite normal. And often these acid coffees are roasted to highlight acidity, almost as drip coffee. However, sweetness has to be present as well, so the roasting curve is adjusted during the roasting process. 

As a general rule you should dial in your espresso using a scale. The first three shots is normally indicating your espresso flow and the appearance of how well your espresso flow/brew.

Your first indicator should be the first drop appearing after activating starting the water flow (pump/bottom).

I recommend the first drop to appear approx. at 6-7 sec. In that way your espressoshot will brew as long a 19-21 sec.

If it appears before 6 sec. I will recommend to grind finer and vise versa if the first drop appears too late/after 8-9 sec or more.
This extraction will give you an espresso with higher acidity. I can also recommend to extract with a higher taget mass as 40-42g liquid in order to it more obtain a medium acidity.  

Dose 21 g dobb shot
Extraction time 19-20 sec
Liguid mass 38 g.
Extraction rate  21/38= 0,55%

The dose is calculated using a 20g porta filter min. dose 19g/ max. dose 21 g. pr double espresso.