I took the drill apart and the gears all came out

well take them back apart and look for your mistake

The gear box of cordless tools is generally and comparatively simple. If you are sure all the parts are present I suggest you engage a member of the family who has the tenacity for jigsaw puzzles to assemble it.

You will find them in the transmission, remove the metal cover and they will be located just under it - sounds like it is either the gear posts or the clutch cams on the nose side of the transmission DeWALT DC925 Type 1 Hammer Drill Parts

Try eReplacementParts.com : Power Tool Parts and Tool Repair

go to eReplacementParts.com : Power Tool Parts and Tool Repairthey had parts diagrams that are helpful.

thumbs back!

I just took mine apart and it appears that you have to replace the entire transmission. It is an easy job and the part is about $34 much cheaper than a new drill. Really, it is one third the price of a battery. Just take the 4 screws out that hold the metal case to the plastic clamshells, slide the front of the drill off, pull the transmission out of the rear of the drill put the plastic piece with the gear indicator on the new trans, if needed, slide the new trans into place, spin the motor if needed to get the output to line up with the planetary gears, slide the front of the drill back into place, rotate the chuck to align the shaft  with the transmission output, put the 4 screws back in, enjoy.

Might want to replace the brushes while you are in there as well. I am going to consider this the dewalt version of an oil change

you never will junk it

first comes the motor, next a gear, then five gears in the planetary gear, then a ring gear, next four gears and another ring gear, finally the gear that goes on the chuck shaft.

You may have put it back together wrong, but you may also need a new transmission. 99% of the power tools I work on where the gears are slipping is caused by the transmission or gear box (both names are used interchangeably).
Good luck to you!
Don-www.toolsandmachinery.com

the transmission comes in assy. all u can do is to match the gear & test every small installed. if it's dw997 see www.erplacementparts.com for your guide how to re-assemble.
if dw988 trans diagram see similarity on how to put it back.