There are two possible uses. If you cast this spell using 1 Action, ... all normal plants in a 100-foot radius become thick and overgrown. A creature moving through the area must spend 4 feet of movement for every 1 foot it moves..."
"...moving through nonmagical difficult terrain costs you no extra movement. You can also pass through nonmagical plants without being slowed by them and without taking damage from them if they have thorns, spines, or similar hazards... In addition, you have advantage....against plants that are magically created or manipulated to impede movement..."
Well then, this one is a little tricky. First and most importantly, "Plant Growth" does not actually create Difficult Terrain with its spell effect, nor does it create magical plants, vines, thorns, etc. It does, however, cause naturally occuring plants to become so overgrown that it impedes or otherwise slows anyone moving through the area, with a magical effect.
When we look at Land's Stride we see that moving through nonmagical difficult terrain will costs you no extra movement. Since this is not difficult terrain, however, this first part does not apply when you move through the effected area of Plant Growth. Then we see that "You can also pass through nonmagical plants without being slowed" , which is where the synergy comes in to play. Since Plant Growth uses magic to cause mundane nonmagical plants and vegetation to overgrow and effect your movement, Land's Stride allows you to move through these plants without being affected. Lastly, since 'Plant Growth' does not offer you a saving throw for you to have Advantage against, the last part of Land's Stride also does not apply here, but it IS important to note that "plants that are magically manipulated to impede movement" is precisely what Plant Growth is doing, even if the plants themselves are nonmagical in nature.