Starting 9am at Bath visiting Stonehenge before 6pm departure out of GAR, time wise reasonable?

Is GAR an airport code? I see it as listed as Papua New Guinea. Did you mean London Gatwick?

Is your departure to the US for which you’d want to give yourselves 3 hours ahead at the airport plus car drop off time.

Were you attempting the airport code for London Gatwick? That would be LGW, just to avoid any further confusion.

You don't state your destination,but if it is outside the UK you need to be at Gatwick by 3 pm. Stonehenge opens at 0930, and takes about an hour to drive from Bath. Assuming you arrive in time for opening, and spend 2 hours on site, you would be leaving around 1130.-1200. It's just under 2 hours drive to Gatwick according to Google Maps. That means no traffic slow downs or diversions, no stopping for bathroom breaks or lunch etc, no missed turns. And then you need to allow extra time to fill up with gas in Gatwick, drop the car, and get to the terminal. By this reckoning, it would be possible, but you would need to be very mindful of the time. I wouldn't be comfortable doing this, but everyone's risk tolerance is different.

GAR? Do you mean Gatwick..the correct abbreviation is LGW? No I would not risk a visit to Stonehenge on departure day. You need to be at Gatwick at least 3 hours prior to departure, plus dropping off the car. Allow I hour to get out of Bath and to Stonehenge, so leave at 9 am get to Stonehenge at 10. At least 1 hours drive. Then visiting Stonehenge…park car, walk to Visitor Centre, walk (25 mins) or shuttle to the stones, walk around the stones. Walk or shuttle back to car park. Leave Stonehenge by midday. Oh, lunch somewhere? Comfort stops? Allow at least two hours for drive to Gatwick. If you are very lucky, and in the unlikely event you are not held up traffic wise - accident or roadworks for example. You might just make it in time but it would be a very stressful day and you would not have had more than a quick glimpse of the site.

Yes, I meant London Gatwick. airport code LGW.

The far better plan would be to stay neither in Bristol or Bath, but in Avebury.

Visit Stonehenge late the previous day. Avebury is part of the Stonehenge World Heritage Site, and is the largest stone circle in the world. At least the stone circle is open from dawn to dusk (if not the associated Alexander Keiller Museum, Manor House and Gardens) so you could get up early, visit Avebury before leaving, then get moving towards Gatwick.

If you were up early enough you could also visit West Kennett Avenue on the way.

Avebury is as important, if not more so, than Stonehenge.

At 7am on Sunday morning, so as quiet as the roads get, aa route planner (a domestic route planner, far better than google maps) shows 1hr47 Stonehenge to Gatwick via the A303, or 2hr13 via the M4.

I live in Bristol and wouldn’t do it personally. If I ever have to fly from Gatwick I stay there overnight. For me there is too much risk of travel problems.