OCS won a new seven year contract last summer covering all landscape maintenance operations.
Landscape maintenance services include all horticulture work (excluding tree work); litter picking; refuse collection; road/path sweeping; cleansing; gritting; playground inspection; and grave digging at Brompton Cemetery.
GMB said large numbers of its members employed as parks gardeners have been in post for many years, some for more than 40 years.
GMB regional officer Gary Carter said: "OCS has served notice on staff to impose the change from weekly pay in the case of St James Park and the pay date in Hyde Park and Kensington Gardens. The changes will have the same effect.
GMB said OCS will withhold two week's pay that its members had already earned, which they will get back when they leave. It added: "OCS will offer the two weeks as a loan that they will deduct over 12 months, putting the employees into debt and wiping out any pay award. OCS will save thousands to the detriment of the employees who will be out of pocket."
Said Carter: "That is the reason members will vote on strike action. The state opening of Parliament is coming up and so too is Trooping the Colour at St James Park. Gardeners from St James Park also maintain the Downing St gardens. It is also the main bedding season. Strike action will have a major impact on these events."
Royal Parks said: "This is not an area for us to comment on."
OCS has been asked for a comment by Horticulture Week.