How can I install a GPS tracker on my fiance's phone without him knowing or being able to find it?

Why would you want to do that? Are you worried he is going to get lost one day like a two year old? Or is he mentally handicapped or has dementia and needs to be GPSed cuz you are worried he will run away and not find his way back home? Or are you starting a marriage out on the wrong foot because there is no trust? Either way you don't want to be your husbands mother or stalker. Now if it is for dementia or something like that then I understand taking on a care taker roll for the man you love. I hope your life together starts out on happier notes. :0)

If you are considering "Tracking" via GPS, your Fiancé's whereabouts . . . Then perhaps it is time to drop the term "Fiancé" . . .

For one of two reasons . . . Either they can't be trusted, or YOU will always be untrusting . . . Either way spells disaster for marriage.

Legally you can not do that to another adults phone.  Its only legal if a person is a legal dependant under the law.  Or if you are a major telecommunications company.  Otherwise he could press charges against you if he ever does find out.  Its also unethical, stalkerish and creepy.  And paranoid.    If you have really good reason to distrust the man, break up with him.  If its an anxiety/paranoia issue -getcounseling and take yiur meds.  If you have no other real reason other than paranoid anxiety to be suspicious then understand paranoid anxiety issues and actions taken like you are suggesting, have destroyed many otherwise good relationships and good people.  Besides, to install invisble tracking app in a way that would be totally non-detectible to him would require skills and knowlege most people dont have.  It will show up one or more somewheres, browser history, download history, payment history, app store que, itunes account, etc. 

sounds a bit.... Fishy to me