That's easy! Leaf worm and giant redworms! Make sure to use cooler to keep live bait stay cool and fresh during hot summer! Crickets is also good bait, too!
Leech is one of my favoeite bait! Almost everything from big bluegill to northern pike! Many bass and catfish, too
Leeches is not cheap! Use it in deeper water to avoid small fish!
Ryan, I agree with all of the previous posters...excellent choices for summer gills. If the gills are going really strong, you can usually get by with plastics vs. live baits or a combination of both plastics and live baits. If you like using leeches, an alternative is Berkely Gulp leeches. They have different sizes (lengths). In deeper water lakes, try dropshotting for gills with the leeches or other baits....a lot of fun. Good luck and tight lines.
Whatever bait you choose ,you might consider that you need to get it down there fast. There are exceptions of course but for deep summer gills I would have a slip float with a heavy shot . This will allow long casts and get down past the dinks fast. Illinoisgiller
Michael is right...you can get pestered enough with small panfish in the water column on the way down if your bait is sinking too slow, Then again, there are times when that slow, natural fall is just what the big boys want, like back in May at Piney Run Lake in MD. However, we have been catching post spawn biggies utilizing slip sinker rigs that get the baits down quickly to the fish. check out the blog on slip sinkers and maybe it will help you out with the summertime/baitfishing gig. Good Luck!