I have a purple gig in my 120. My pH fluctuates between 8.0-8.4. The ideal pH for gigs is 8.3. However, it's more important to provide them constant parameter rather than trying to chase a number. IMO a range of +/- .5 is fine.
I also have a green gig in a 10 gallon tank. It was supposed to be temporary but is now going on 2 months and it's fine. It shares the tank with two dozen baby clowns. Water parameters are far from perfect. While I don't advocate this, it just goes to show that gigs are very hardy once they are acclimated and healthy.
Keep in mind that gigs live in tidal areas where they are exposed to lots of sunlight, even partially if not fully exposed to the air during the day. They also have to deal with typical life in a tidepool -- constantly changing, turbid water where salinity can vary throughout the day.
Those of us who have gigantea for a long time know that they are almost bullet-proof. Owner error -- such as a kalkwasser overdose -- are the main causes of the death of a gig that's been in a tank more long than 6 months.
The key when acquiring a specimen is finding one that hasn't suffered much shipping stress and responds to antibiotic treatment.