In Gmail, click the ⚙ gear (top right) → See all settings.
On the General tab, scroll to Signature → Create new → name it, e.g. “Sustain Health”.
Click in the signature box and paste (⌘V on Mac, Ctrl+V on Windows). Delete your old signature if there is one.
Under Signature defaults, choose it for both new emails and reply/forward.
Scroll to the bottom and click Save Changes.
If you send from a shared address (e.g. reception@ or accounts@), pick the signature for that address too in step 5. The Gmail phone app only allows a plain-text signature, so it won't show the logo.