It says you can buy it for 5 sp but doesn't list a weight. All it does is give a description" "When you press a signet ring into warm sealing wax, you leave an identifying mark." So how would I buy a gallon of this stuff if I don't even know how much 5 sp will get me?
The Wikipedia article on sealing wax is a good overview of what sealing wax is and how it’s used.
For our purposes, I just want to draw your attention to the unit described. Basically, when you bought real-world sealing wax you got a stick of wax about the length of a pen and about as thick as a person’s thumb, or the equivalent quantity of loose granules of wax. This is very approximate, and obviously exact measurements would vary by the seller, but this is the general scale of a real-world unit of sealing wax.
Since D&D approaches world details by taking our understanding of the real world as a base onto which we can add fantastic elements, we can just assume that D&D-fantasy sealing wax is sold in the same approximate units that were convenient for real-world people.
A stick of wax about the length of a pen and the thickness of a thumb is, let’s approximate, about 5 inches by 1 inch square, which is a volume about 5 cubic inches or, rounded up to be generous, about 3 oz of solidified wax. Let’s assume the wax doesn’t expand appreciably when it melts just so we can get a ballpark number. That means a US gallon (defined as 231 cubic inches) can contain about 46 sticks of our estimated sealing wax unit.
So a gallon of sealing wax would run you about 230 pieces of silver, or 23 gp.
Again, this is very approximate, since “sticks” of sealing wax isn’t an exact volume to begin with, or even standardised between different colours or qualities of wax, or from seller to seller. Somewhere in neighbourhood of 20–30 gp is about the idea. Ask your DM to pick a more exact price when your PC goes to buy the stack of sticks from a merchant, so you can move on to melting gallons of sealing wax onto things.