Ambrose Phillipps' design of the Triumphal Arch at Garendon Park was influenced by the Arch of Titus in Rome. This is evident from a sketchbook put together by Phillipps, which based the design on this specific ancient Roman source.
For more details, you can refer to the original sources:
Which historical family was primarily associated with the ownership of Garendon Hall?
What was the fate of the Palladian follies designed by Ambrose Phillipps on the estate?
What architectural styles influenced the design and construction of Garendon Hall?
What architectural style was the revised Garendon Hall supposed to be in?