Napoleon was quite specific for why he chose to take Gribeauval guns to Spain.
Spain and France being Bourbon countries both used the Gribeauval system, it being used in all bourbon kingdoms. Thus when in Spain, Napoleon could use captured ammunition in the guns he took to spain, rather than having to transport ammunition alway the from France. It seems Napoleon understood logistics to feed his guns, but no his men or horses. But also relying on captured ammunition fits in with the mentality of travelling light and use what ever we can find and lack of logistics as mentioned in earlier threads about the French in SPain and portugal inparticular
Ammunition dumps were established for the siege trains and troops operating in north france, but the bulk of the ammunition used in spain was captured spannish. In addition a lot of the guns used would have been of spannish origin given the large losses of ordnance by the french and spanish in this theatre.
AnXi 12-pdr had a very slight difference in calibre to the Gribeauval one, so in theory AnXI ammo could be fired from it. the 8-pdr and 4-pdr could not use AnXI ammo. However their were huge stock piles of this ammunition for these calibres which again influences why it took from 1803 to 1810 to fully introduce the AnXI system. Cost of using existing ammunition rather than making new was a key factor.
Given that in potentia AnXI ammunution was more effective than Gribeauval, I would suggest this is taken into fact for war games rules in terms of range and effecr. AnXI guns in general had longer ranges. I hope to publish on AnXI artillery in the near future, as I believe it is a subject that is need of serious clarification.