Is a Deka 8G8D different from an MK 8G8D?

No. East Penn Manufacturing builds the battery in the United States; Deka is East Penn's own brand and MK Battery is the distribution arm that carries it into mobility, marine and renewable channels. The unit that ships is the same 8G8D regardless of which name your paperwork uses, and the label may show either badge depending on the production run.

Should I buy the gel 8G8D or the AGM 8A8D?

Match the construction to the duty. The gel's immobilized electrolyte tolerates deep, repeated cycling and extreme temperatures better, which is why solar and mobility installs specify it. The 8A8D answers with a higher 20-hour rating, 245 Ah against 225, in the same group size. If your bank discharges deep every day, the gel earns its keep; if the duty is lighter and you want maximum rated capacity per tray slot, the AGM case is easy to make. A specialist will confirm against your load profile.

What does the -76°F rating mean in practice?

It is the published floor of the operating range, and it means a cold-soaked battery will still deliver current where a flooded or AGM unit may not. Capacity at that temperature is a fraction of the 77°F rating, as with every lead-acid chemistry, so cold-climate banks are sized with margin. The practical takeaway: an unheated shed, hull or cabinet in a northern winter does not disqualify this battery.