My dishwasher leaves about 2-3 gallons of water in the bottom at the end of a completed cycle. I have already checked the air gap (put a 3 ft cable tie down to confirm there was no blockage). I also drained all the water and took the hose off (check valve) under the dishwasher to confirm no blockage. I can’t figure out how to easily get to the drain in the dishwasher to confirm no blockage there. Are there any other diy tips before I call a repairman? I have 4 kids and loosing my dishwasher is killing my already not-so-smooth hands 
Here are your parts
Replacement parts for WHIRLPOOL GU1200XTLS3 Undercounter Dishwasher | AppliancePartsPros.com
Here is the tech sheet
http://www.servicematters.com/docs/wiring/Tech%20Sheet%20-%208537797.pdf
Am not sure how you checked the air gap.
You have to check it from the machine to the air gap and from the air gap to the house plumbing.
Check to see if the drain pump is running.
If it sounds/feels like it is then I would undo the drain hose at the air gap and try to drain it into a pail. Be sure to hold the hose securely in the pail and put a towel etc. over it so it does not splash all over. If it now drains your problem is either the air gap or a blockage (air gap to house drain.
If drain pump does not seem to run then check to see if it is getting power when it should.
If it is and tthe pump is not running it is probably toast or has something jamming it.