Hi Grant - there is no "gift tax" in Canada.
The Canadian tax code includes a set of rules which are designed to prevent people from transferring assets between spouses or to minor children in order to avoid paying tax. These are called the attribution rules - because they cause income from a property held by one person to be attributed back to another.
However, because you are not a minor, and because the gift is not being made from one spouse to another, and because there is no general "gift tax" in Canada, there is no tax consequence for you or your fiancee associated with your fiancee's parents giving you land.
HOWEVER, your fiancee's parents will be deemed to have disposed of the property at fair market value, and they may be subject to a capital gains tax on the land as a result of their disposition (by way of a gift to you). Here
is the Revenue Canada interpretive bulletin on gifts of property.
Also - without making this response overly long - if your fiancee's parents decide instead to "sell" you and her the property for a very modest sum (sometimes people do this) - there are negative tax implications for you. Outright gifts are much better than sales for less than the fair market value.
Finally - when you build the house on the land, the house and land will together form your matrimonial home (either on the date of your marriage, or on the date when your relationship becomes a common-law relationship, or on the birth of a child), and your financee will lose any separate claim she has to that gift of land in the event of a marriage breakdown. I'm not suggesting anything here, believe me! However, I do think it is useful to know what the consequences of these kinds of decisions are.