Strain first. $\varepsilon = \dfrac{\delta}{L}$ — epsilon equals delta over L. $\delta$ (delta) is the change in length, how much the specimen grew under load; $L$ is the original gauge length, the marked span the instrument watches; $\varepsilon$ is the strain, the growth per unit of original length. Both lengths must be in the SAME unit before you divide — that is the entire difficulty of this formula, and it catches somebody every year.

Then the price. $E = \dfrac{\sigma}{\varepsilon}$ — E equals sigma over epsilon — is Young's modulus: the stress it takes to buy one unit of strain. $\sigma$ is the stress in the specimen, $\varepsilon$ the strain it produced, $E$ the material's stiffness. Steel lands near 200 GPa, aluminium near 70, copper near 110 — and those numbers belong to the ALLOY, never to your particular bar. A thicker bar is stronger; it is not stiffer per unit of stress.

Because strain is a bare ratio, $\sigma / \varepsilon$ comes back wearing the stress's own unit. Feed it megapascals and megapascals come out; the gigapascal on the datasheet is one division by a thousand away, never a different quantity.