The report will examine the series of increases that have hit the project and seen its cost jump to £400m, 10 times the original budget. It will also consider whether the scheme achieved value for money and how the project was managed.
The probe is a follow-up to an Audit Scotland report on the Scottish parliament building published in 2000, to which G&T also contributed. The new report will cover the period 2000 to 2003.
G&T's work on the beleaguered Enric Miralles-designed scheme could also feed into Lord Fraser's public inquiry into the project, which is due to reconvene next month.
Auditor general Robert Black said last autumn that the interim findings from the report would be published next summer, which is when the scheme is due for completion.
Around 50 members of the project team were forced to work over the Christmas period to ensure that the summer deadline is met.
Audit Scotland originally tendered for the job last October and said it "urgently needed" to make an appointment by the end of last year.
A statement by the organisation confirmed G&T beat off five rivals to win the commission. The statement said: "We selected G&T as the best value-for-money proposal after assessment of each tender. This included appraisal of the quality, scope and the price and affordability of all six proposals."