If you’re bringing your own libraries into Gatling’s classpath, it’s your responsibility to deal with dependency versions clashes.
Here, it seems opensaml still depends on slf4j 1. Basically, the Java ecosystem is currently split 50/50 between slf4j 1 and 2.
Somehow, your build tool (maven?) is picking opensaml’s slf4j 1 over our slf4j 2. You must then make sure to have the proper logback version, see Logback Dependencies