I have decalcified my Gaggia Synccrony twice using a product from the grocery store called dip-it and it is still saying to decalcify the machine

First of all - Dip-it is a detergent to remove rancid oils from coffee cups and coffee makers. That is not the same thing as a decalcifier or descaler. Dip-it was never meant to go through your boiler and pump and valves and all the other parts inside your Gaggia.

I'm quite sure that if you read the Gaggia manual it will tell you to use a special descaler such as Dezcal or Durgol.

Some people say use white vinegar - the manual specifically says not to use vinegar.

The machine is pretty expensive - it's worth buying the right product to descale it.

The manual should walk you through the steps to descale which will clear the error message once all the steps have been completed. I don't own that specific model, but typically you add the dezcal powder to very warm water in the water tank. Then start the descale process which will prompt you to open the steam valve and pump some water through the steam valve (into a container you place under the steam wand to catch it). Typically it will cycle a little water then pause for several minutes while the descaler disolves the calcium build up in the boiler - then it will pump a little more though and then wait etc. etc. Depending on your machine it may pump water out the coffee spout as well as out the steam wand. If so, the machine should prompt you to tell you to open and close the steam valve.
After it has run all the chemical through the machine (usually after it has emptied the tank) it should then prompt you to rinse. You remove the tank and wash it out and fill it with clean drinking water and then let the machine continue on the rinse cycle which will pump all the clean water through the unit. So you need to be on hand to catch all that water and dump it in the sink. When you are all done, run a few blank shots and make sure the water tastes like clean drinking water. The descaler is mostly citrix acid (like lemon juice) so if the water tastes sour like lemons then it isn't completly rinsed yet and you may want to pump more fresh water through the machine (pull blank shots or pump hot water through the steam wand) until it is fully rinsed.
Once the machine completes the rinse cycle it should stop prompting you that you need to descale.