The chords uses all strings but the 5th one, the A string, how do I strum strings skipping the A string?
You don't skip the string. You strum all the strings and use the fretting hand to mute the string(s) that you don't want ringing.
In this specific case you can mute the 5th string with the same finger you are using to fret G (3rd fret 6th string). You press G like you'd normally do, and use the bottom part of your finger to mute the 5th string. If you position your finger correctly, you can press G and mute A with the same finger, while letting the other strings ring.
This way you can strum really hard and do stuff that you'd normally do only with open string positions. You can also do percussive stuff when muting all or many strings.
It's a very useful skill to have, and pretty much essential in some genres and styles. It's hard to describe with words, youtube has many videos on the subject with different approaches, like this one:
As you can see it's used a lot in funk, but it has uses everywhere. It's a rhythm guitar concept, so it's not the most popular one.