The central point of Psalm 100 is found in the center:

Our LORD (Yahweh) is God  (Vs 3)

Because that is true...

The invitation is offered to all to be happily united with our God.  (Vs 1)

And because our Lord is God & the ultimate provider & all are invited to unite with Him...

Our Worship/Service will be offered with spontaneous enthusiasm!  (Vs 2)

And because our relationship to our Lord brings us joy and gladness...

We are a thankful people!  (Vs 4)

And so it shall ever be!  (almost a P.S.)

God's love (mercy) is for always & His truth is truth in every generation!  (Vs 5)