How does the transitioning to another Grand Company work?

I know you lose your rank, but wanted to know what else you lose. Do you lose all your allied seals, GC seals? Do you have to do the rank quests again? Grand Company log?

You don't lose your rank at all, when ever you wish to transfer over back to your old GC, your old rank will still be there for that GC. You start out as a new GC rank in a new GC, you'll have to pay a 1 time fee of 50,000 gil. And you must wait 15 days before you can transfer to another GC again.

I believe you loose your pvp rank too. Though it might go back if you change back.. Also you cant seem to wear GC clothing anymore if you have left the GC. (personally i think thats kinda silly of SE. Letting us wear the clothes we purchased should be okay and would be more of a incentive to continue ranking in multiple GC's)

Hm thanks. Though I'm more concerned about my Grand Company seals and Allied seals. Will all that be lost in starting in a new GC?

You can't use the seals from one grand company to pay for something that belongs to another one. You keep all GC/Allied Seals as well as your rank with that particular Grand Company, but you will be unable to access something from the new Grand Company until you've risen your rank with that specific faction to a satisfactory level. This includes the quests and hunting log requirements for promotion.

As a side note, don't change grand companies while you're doing Atma Books because some of the Leves are Grand Company leves that require you to be a Chief Sergeant with your current faction in order to undertake.

Nvm googled it and you keep allied seals

I think the very first time you ever change GC is free, but 50k gil for every change after that. You will retain your GC rank,PVP,seals etc but be unable to access any of them until you switch back.
So for the GC you just joined, yes, you start out at zero seals, and have to rank up from the very beginning again.
Also, any items you have that are glamoured with a GC specific item (ie: Allied Seals GC gear), will show their default, non-glamoured skin. And chocobo will be unable to equip the old GC bardings.