Let's be honest: HDMA gradients are, due to their nature, static to the screen. But other games somehow manage to manage to make them scroll (well, give the illusion of scrolling as remember, they're static by nature) without using any background! Don't worry now, it's time to officially have that kind of code in SMW (at least a proper release since years)!
Just so you know, I recently discovered that using this code in your levels will mess up one row of your BGs, and that's due to the free RAM used by the code! MFG himself told me this in a DM. Maybe use the OW event restore patch if you plan to use the code?
Edit: After applying it, go to 'ScrollHDMA.asm' and change '!FreeRAM_RG_Ptr' and '!FreeRAM_B_Ptr' to something like $7F0000 and $7F0007 respectively. No need to convert the addresses if using SA-1.