It is not permissible for you to work as a gardener in a non-Muslim graveyard, for two reasons:
Planting flowers, trees and so on in graveyards is something that is not prescribed in Islam; if this kind of work is not permitted in Muslim graveyards, then how about in the graveyards of others?
Shaykh Muhammad ibn Ibraaheem ibn Aal ash-Shaykh said: With regard to planting trees in graveyards, it is not permissible, and it is an imitation of the Christians who make their graves look more like gardens. So they must be removed and the irrigation systems that have been put in place to water them must also be removed.
End quote from Fataawa wa Rasaa’il ash-Shaykh Muhammad ibn Ibraaheem (3/200)
Shaykh ‘Ali Mahfooz – one of the scholars of al-Azhar – said:
It is not permissible to build dwellings, or to plant gardens, or to set up irrigation systems, in graveyards. All of these things are reprehensible innovations and transgressions against the rights of the Muslims, and it leads to negative consequences that are not unknown to anyone who has insight.
End quote from al-Ibdaa‘ fi Madaarr al-Ibtidaa‘ (p. 181)
The Council of Senior Scholars issued a ruling stating that it is haram to plant gardens in graveyards. Among the things they said in it is:
The Council has unanimously determined that it is haram to do any work in graveyards, and they should not be planted with trees, or have lighting installed, or have anything done to them that is aimed at beautification, so that they may remain as they were at the time of the righteous early generations, and so that the graveyards will be a source of exhortation and reminders.
End quote from Nayl al-Ma’aarib by Shaykh al-Bassaam (1/ 512)
See also the answer to question no. 14370
The disbelievers are punished in their graves, and we are prohibited in Islam to remain in places where punishment is occurring, because the Prophet (blessings and peace of Allah be upon him) said: “Do not enter upon these people who are being punished, unless you are weeping. If you are not weeping then do not enter upon them, lest there befall you the like of what befell them.”
Narrated by al-Bukhaari (433) and Muslim (2980).
What should be done when passing by their graves is to weep, feel fear, and express one’s need for Allah.
Imam an-Nawawi put the hadeeth quoted above under the following heading:
Chapter on weeping and feeling fear when passing by the graves of wrongdoers and the places where the punishment of Allah came upon them, expressing one’s need for Allah, may He be exalted, and warning against heedlessness with regard to that.
End quote from Riyaadh as-Saaliheen (p. 297).
Al-Qurtubi said: Based on that, some of the scholars stated that if one enters the graveyards of the disbelievers – if one enters any of those places and graveyards, he should do so in the manner described by the Prophet (blessings and peace of Allah be upon him), i.e., one should try to contemplate that, feel fear, and hasten to leave.
End quote from Tafseer al-Qurtubi (10/46)
Ibn Rajab al-Hanbali said: This hadeeth clearly states that it is not allowed to enter places where people are being punished, unless one is in a state of complete humility and contemplation, which means weeping from fear of Allah and of His punishment which befell those who are in that place. If one does not enter in this manner, there is the fear that the same punishment may befall you as befell them.
This indicates that it is not permissible to live in such places or to stay in them. This was stated clearly by a number of scholars, including al-Khattaabi and others, and was stated clearly by Ahmad.
End quote from Fath al-Baari (3/237)
As-Safaareeni said: This hadeeth encourages us to avoid the places of Allah’s anger and wrath, and to keep away from the graves of wrongdoers, and their dwellings and the places where the punishment befell them, if we are heedless of what befell them of the wrath and punishment of Allah.
End quote from Sharh Thulaathiyaat al-Musnad (1/51).
Ibn al-Qayyim (may Allah have mercy on him) said:
Whoever passes by the land of those with whom Allah was angry or who are being punished, it is not appropriate for him to enter it or stay there; rather he should move on quickly and cover his head with his garment until he has passed it, and he should not enter upon them unless he is weeping and contemplating.
An example of that is when the Prophet (blessings and peace of Allah be upon him) travelled quickly in the Valley of Muhassir, between Mina and ‘Arafah, because that is the place in which Allah destroyed the elephant and its companions.
End quote from Zaad al-Ma‘aad (3/488)
To sum up:
Working as a gardener in the graveyard of the disbelievers is a prohibited kind of work that it is not permissible for a Muslim to do. What you must do is look for a different job that is permissible.
And Allah knows best.