Hi Andy,
I realize this may arrive too late, however using BAL GT1 with Grout 3000 would be OK. For obvious reasons we'd prefer Flexmaster to be used, but where it is needed, GT1 will be suitable.
I don't believe GT1 to be a PVA, although it is a form of enhanced liquid polymer that will increase the adhesion properties and flexibility of the grout, or product it is added into. I am not sure of the dilution rates of GT1 however so would suggest adding small amounts at a time to get to the consistency you would usually see from using Flexmaster with Grout 3000.
Of course there is no issue with you using Flexmaster in all applications of Grout 3000, however our usual advice would be to use Grout 3000 & Flexmaster in any application where Ultimate adhesive (S2) has needed to be used. Steel substrates, T&G Floorboards, Fibreglass etc. In all other appliactions, Grout 3000 can be used as it is.
I hope this helps.