Watch Order

Others have answered your question but let me explain more in-depth.

There's the Trigun manga, which was continued under the name Trigun Maximum when i switched manga magazines.

The first anime adaptation from the 90s adapted some parts of the manga, but added in a lot of its own stuff as well. It's great, but it's quite slow-paced and very 90s. There was a sequel movie, Badlands rumble, which I haven't seen and actually haven't heard a lot about.

Then there was Trigun Stampede, which is a new anime that is unrelated to the original. It's much faster-paced, and while it does take a lot from the manga, it's its own original story based on it vs being a straight adaptation or a sequel to the old one. This anime, Trigun Stargaze, is a sequel to that and takes place...I think it was 2.5 years later? So Stampede is the only essential viewing here.

Personally, I think the original is much better, but the new one is much more accessible, with faster pacing and more action, and is made to be enjoyed by both new and existing fans alike. The only issue is that watching Stampede will spoil some twists from the original, while that same information is presented as more like...normal development in the new one, because they assume a lot of fans will already know it coming in. But if you're not that worried about the old one, then go ahead and start with Stampede and Stargaze. And even if you do still want to watch the original after, even with spoilers, there's still a lot to enjoy.