The tablespoon is a unit of volume in the US customary system and uses the symbol Tbsp.
In the kitchen, the tablespoon is the larger of the commonly used spoons served at the table and represents 1/2 US fluid ounce or, expressed in SI / metric format; 14.78676 ml.
This is often expressed as equivalent to 3 teaspoons (3 Tsp) although strictly speaking it is a little under the size of 3 metric teaspoons (5 ml) at 4.92892159375 ml. It was established to assist normal kitchens with recipes without the need for specific measuring equipment or devices. Although many spoons differ in sizes (and very rarely is a spoon filled accurately) it allowed for a decent "catch-all" and gave a "good enough" result most of the time.
The gill is a unit of volume in the imperial unit system and uses the symbol gi.
It represents 1/4 of a pint - so in this instance, it is 1/4 of an imperial pint - therefore measures exactly 142.0653125 ml. It's also equal to 2 imperial cups and 1/32 of an imperial gallon.
It is pronounced with a "J" sound at the beginning - and it actually part of a humorous in the nursery rhyme 'Jack & Jill'.
In the UK it was used throughout pubs as standard measures; for example spirits were served as 1/6 gill as standard. In Scotland it was 1/5 gill and in Ireland 1/4 gill. It's still used in the Republic of Ireland today.