Even though you can mix White Gull with identical secondary substances (like Rubedo), it will not carry the effect over to the potion you have mixed. So you best mix it with those ingredients that lack any secondary substance, unless you plan to simply drink it, then you can benefit from a dominant secondary substance.
Try to have a small stack of White Gull in your inventory while you play. Sometimes you will find yourself in need of a potion that needs high or top quality alcohol. Especially in the later parts of the game, a small stack can be both a time and life saver since you may not always find the needed ingredients and alcohol.
A stack of White Gull tends to take up less inventory space than the often three or four different alcohols you might otherwise be carrying. It may be useful to convert any available potion bases to white gull immediately to save space, even those that would normally serve as top quality bases themselves.
There are two potions of White Gull in the armory on the second floor of Kaer Morhen in the Prologue.
White Gull is sometimes used by locals as a palliative potion, to ease the suffering of the sick and dying. An old woman beggar in the Outskirts makes such a request of Geralt. If he obliges, she rewards him with the Eternal Fire signet ring.
Be careful when using alchemy as the game may sometimes use White Gull to brew more White Gull, potentially wasting perfectly usable inventory.
It takes 40 White Gull's to brew all "Superior" recipes so it's best to stock on the ingredients as early as possible. White Orchard is in fact an excellent place to acquire all ingredients during the prologue, as arenaria is abundant there and Elsa sells all 3 required alcohols. In fact she is the only merchant in the base game to so, however, after the prologue she will no longer be available, so it's best to deal with her while you still have the chance.
Redanian herbal can be found as random loot so if you can't afford to buy all 3 you can skip it.