[George from New England]

I’m not a scientist. The article talks about “ranging from nearly 4 Ga ago to 3.7 Ga ago.”

I am clueless, the article doesn’t teach us lay people what this ‘Ga’ unit is at all.

[Boogieman]

That’s an abbreviation geologists use which means “billion years”.

[Ken H]

It's just a wild ass Gauss.

(thank you, thank you very much!)

[Rodamala]

Gauss is a measure of a magnetic field. It measures the magnetic flux density of the field at a particular point in space.

[UnwashedPeasant]

"the article doesn’t teach us lay people what this ‘Ga’ unit is at all."

It is a standard unit of measure:
1 Ga unit = Richard Simmons
2 Ga units = Algore
3 Ga units = Obama

It als has this meaning: Gigaannum or Ga, a unit of time equal to 1,000,000,000 (one billion) years.

[Yashcheritsiy]

I am clueless, the article doesn’t teach us lay people what this ‘Ga’ unit is at all.

Ga = "giga-annum" - billions of years. A mythological unit of time not much different from the "in illo tempore" (once upon a time) of the fairy tales.