The 9th note in a minor chord always brings out tension, so even though the very first chord here is GMaj7 — a smooth chord — the bass note quickly going down to E to make the whole 1st section stay in Em tonality. Then it goes to A but again the F# persists to give it both ambiguity and continuity.
Here is an interesting riff, I can hear the opening single-note part being rock, blues or something laid-back like reggae. It’s just a G octave, but with a swinging rhythm and a little ornamentation it can be this catchy. Watch out for the Bb to D jump down on the turn around — everything else is a step-wise G minor pentatonic, but that jump gives is enough momentum to go back to the beginning. I played the C to Bb response section with modern voicing, but playing a more traidional C or Bb bar chord will make this sound more straight-up classicl rock or blues rock.