I remember this story and I'm a football fan so I might be able to help you out.
First of all, you may know that homophobia has deep roots in some groups of football supporters (a Google search gives plenty of documentation on that matter). A lot of supporter songs can be interpreted as homophobic, and it is subject to big debates in French football lately.
Last year, fighting against homophobia in football was a very hot topic and several measures were taken by officials (such as making the players wear rainbow-coloured armbands). Some supporter groups felt targeted and did not like it at all. At Nice, who is renowned for having some of the most virulent supporters (or, as we call them here, "ultras", as in ultra-involved in the club and the stadium), two banners were deployed, both targeting homosexuals by using word plays:
Bienvenue au groupe Ineos : à Nice aussi on aime la pédale !
This happened at the end of August, 2019. The OGC Nice football club had just been bought by the Ineos group, which is famous for being the biggest (and richest) team in cyclism. The word play is with the word "pédale", which means the pedal of a bicycle, and which is also an insulting word to describe an homosexual man.
Des parcages pleins pour des stades plus gays
The context here is that in France, groups of supporters are very often forbidden to go to their team's games when they play away from home, for security reasons (to avoid fighting between different supporter groups) and as punishment for rules violations (using smoke bombs, etc). In stadiums, supporters of the visiting team generally have a dedicated zone, called a parcage.
The word play here is to say "if the parcages are full, stadiums will be more lively", using the literal definition of the word gai in French. It is pronounced, but not written, the same way as the word gay, which has the exact same meaning as in English. So their goal was to protest against parcages forbidding, while sending a message about all the homophobia topic.
I hope it's clearer now!