You can certainly open up a claim with your insurance company and see what they say. Even if it’s not totally clear this truck hit your car because there were no witnesses, they may be able to advise on next steps for what could be considered a hit-and-run.
Likely your gap insurance would not cover the difference if you take the lower payout. Most gap plans are written in a way that requires you to take the totalled vehicle payoff and apply it to the loan first.
If you no longer have the vehicle and you don't intend on insuring a new vehicle with the same company, you should be able to back date cancel it to the day after the accident. Your best bet is to call your carrier’s customer service line and see what they can do for you.
Unless otherwise requested, changes on insurance policies go into effect right away. As long as you requested the change sometime before you began your trip, the deductible change is in effect as soon as you request it from your insurance company.