Guild Wars 2

If I gift a booster to Person A at full price..

And then I gift a booster to Person B, do I get the 40% discount?

Or does the discount have to be spent on myself?

No.

Only if you buy the same booster for yourself. Note that the discount is still applied regardless of which size quantity you buy. That means if you buy a single booster as a gift for one player, you can buy the 5 pack version for yourself at 40% off (or gift a key and buy the 25 key pack for yourself).

Gift a 2nd item, you qualify for another discount.