ServerPlayerEntity extends PlayerEntity. No idea where which one is used, but you can run an instanceof and try casting. And yes, changing the method signature would invalidate the @Override, but basic Java is normally outside the scope of this forum.
You can try:
if (player instanceof ServerPlayerEntity){
ServerEntityPlayer serverPlayer = (ServerEntityPlayer)player;
glide(serverPlayer);
}else {System.out.println("player is not a ServerPlayerEntity :(");}
and feel free to look up appropriate Java tutorials for casting and polymorphism.
But like I said, I'm not sure exactly when PlayerEntity is or is not an instance of ServerPlayerEntity.