Question About Fishing Guild GP!

I always been told that fishing is apart from other craft, that you can take it to 100 even if you have another craft higher than 60. But what about GP? If I begin to turn in items for Leathercraft GP I will lose all my fishing GP? Like they let me take fishing to 100 too but they don't let me get the good GP items that come with it if I need GP in another craft? :/

yes, you will lose GP from other crafts if you trade items for fishing GP.

You can only have 1 GP contract at a time.

You can get all the good items for fishing, but not at the same times as getting LeatherCraft items.
After you get your LeatherCraft items you can start the new contract with the Fishing guild, at this point you will lose any Leather GP you have left over, but not the items you have already bought (including Key items).