Wiring sub-panel

Hi all,

I am planning on wiring a sub-panel for a GFCI located in the yard.

The panel i'll be using is a Square D 70 amp panel Square D Homeline 70 Amp 2-Space 4-Circuit Indoor Flush Mount Main Lug Load Center with Cover-HOM24L70FCP - The Home Depot

This will be fed from a 20 amp breaker with 12/2 wire from the main panel. There will not be a second breaker added to the sub-panel so I don't think I need the 12/3 wire. This will be a 20 amp circuit with 20 amp breaker (in sub-panel) and 20 amp GFCI.

Can someone tell me if the following wire configuration is correct? Oddly there is no diagram provided with the panel.

Wire from main panel:
Hot wire to main feed in sub-panel
Neutral wire to neutral buss bar
Ground wire pigtails with ground wire to GFCI

Wire to GFCI:
Hot wire to 20 amp breaker in sub-panel
Neutral wire to neutral buss bar
Ground wire pigtails with ground wire from main panel

There is no ground bar in the sub-panel, so I figure the two grounds connect together. There is a green screw that when tightened, grounds the neutral buss bar with the panel housing. My main concern is the ground wire. I have the same setup for an existing GFCI with the grounds connecting together then attached to the panel mounting screw.

Any help would be appreciated!

Wiring sub-panel