AD scaling on attack enhancers is always a mistake for balance plus Galio is a mage and this will just noob-bait players with building the wrong things, it is also easier to balance for the longer term with ability power ratios since there is no double up on the same stat scaling (i.e. attack damage giving your attacks more damage plus a ratio that gives your attacks more damage, this is always a mistake). Give Galio an AP ratio for the passive.
Now that I have seen the tooltip, the preview is very misleading, make the damage (100%AD)(X%AP)(X%MR) as magic damage in an area.
That preview tooltip is so misleading, but the problem to having a visible AD ratio to indicate damage is converted to magic damage is that new players and bad players think he has an AD ratio and you will see people doing stupid builds like full AD Galio in a ranked game. I learned to give bronze-silver players less credit than you think because common sense is just not there in the lowest elos - they still think Twisted Fate as an AD ratio and cannot figure out its his attack becoming magic damage.