If you want to convert your D-tuned Seagull Merlin M4 to a G-tuned one (using the same string gauges that are factory-shipped), you'll need a different bridge saddle.
And of course, if you're looking to replace a lost or broken one, here you go!
But... WHY?
Why do you need a different saddle to tune to G? String thickness. G Seagull Merlins are shipped with lower gauge (thicker) strings than most, so the spacing at the bridge is slightly different. If you try to simply change your D over to a G without a different bridge you'll likely notice times where it sounds out of tune. Potentially at every fret except an open string.
Caveat: If you choose to use G-set strings that are the octave above the D tuning then you probably won't need a different saddle. Note that that's not the string setup that Seagull means when they refer to a G Merlin, but it is the register that pickin' sticks and Strumsticks are accustomed to tuning to.