Gig posters are not album covers. Theses are the posters that drive fans to concerts.
Gig posters should reflect the music they represent. Unlike album covers bands have control who does the gig posters. This is very important as gig posters are directly responsible for band's revenue streams. Concerts are how modern bands make money.
Usually, the artists are fans of the bands attempting not only to make a poster to promote the music they love, but also to create a collectable representation to inspire people to buy tickets. These posters are to influence people to listen to the band.
You should be able to tell the style of music from the poster.
These posters can be influenced by a song, but should represent more than just that song.
The posters should pop and be easy to read.
These posters should have energy and excitement to reflect the mood and voice of the music and to entice potential ticket buyers.