The rank or strictly appointment of Bombardier moved your man up one step from Gunner. Gunner is equivalent to an infantry Private, Bombardier to Lance Corporal. He would wear a single stripe or chevron on the upper sleeves and would have some degree of responsibility within his gun crew. After WW1, Bombardiers wore two stripes and were equivalent to Corporal.
The Royal Garrison Artillery was the branch of the Royal Artillery which handled larger and more static guns than those you may have seen on the Queen's Birthday Parades and other ceremonial duties. They would be postioned some way behind the trench lines and would be used to counter enemy artillery and to destroy their trenches. The enemy would of course be aiming at them too, so being behind the lines was no escape!
The Long Long Trail website will tell you more:
http://www.1914-1918.net/The medal index card shows that he earned the Victory and British War Medals, but not the 1914-15 Star, which probably means that he did not serve in France and Flanders until say 1916.
There is a small chance that his service papers have survived - over 60% were lost in the 1940 Blitz. If they have, you may be able to download them via Ancestry, or see them at the National Archives.
The 'Roll' and 'Page' columns refer to the medal rolls themselves, only available by visiting the National Archives or using an agent, but there may not be much more anyway.
I have just looked at the LLT and see that the 230rd Siege Bty arrived in France on 18 Jan 1917. There is a picture of a howitzer in action, but there were other guns used.
Others may be able to guide you towards War Diaries for the unit, as I am not familiar with them.
Hope this helps.
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