He will hand control of construction to Bill Hocking, who has been managing director of the division since joining from Skanska UK last September.
His remaining responsibilities for the housing arm will be transferred over the coming 12 months.
Gillespie announced he would be retiring as the housing and construction group unveiled record half-year profits up 24% at £53m.
The results included exceptional £3.7m costs covering restructuring and redundancies associated with the integration of Miller Construction on top of £3.4m booked the previous year.
Galliford Try’s housing business Linden Homes delivered an operating profit of £61.5m as margins rose from 15% to 17%. But investment in land saw net debt rise from £36m last year to £96m.
The firm said it now planned to open a further Linden Homes business in Yorkshire and a Galliford Try Partnerships business in Bristol to take advantage of rising demand.
Construction division profits edged up from £5.9m to £8.5m as the firm worked through legacy contracts, edging margins up from 1% to 1.2%.
Peter Truscott, chief executive, said: “The construction market continues to improve, although at a slower rate than anticipated.
“We have nevertheless further improved our order book to £3.7bn, with good visibility on projected revenue for this and the next financial year. Build cost increases have moderated and availability of skilled labour has improved across all regions.
“Linden Homes has made strong progress on margins as well as increased outlet numbers and unit sales per outlet.
“Our Partnerships business continues to see good demand for contracting in the affordable housing market, as registered provider clients assimilate the recent changes to their income profile; and importantly the mixed tenure development business is growing strongly.”