Are gametes diploid or haploid?
Actually there is some confusion here, and that's quite excusable, because it's extremely common reading that monoploid and haploid are synonyms and have the same meaning. However, they are different terms. According to Hartl and Ruvolo (2012):


  The potential confusion arises because of diploid organisms, in which the monoploid chromosome set and the haploid chromosome set are the same.


As we, human beings, are diploid organisms, it's easy to see why haploid and monoploid ended up being considered as synonyms. 

However, a more precise terminology would be:


Monoploid: the total number of chromosomes in a single complete set of chromosomes (this does not change whether we are talking about a somatic cell or a gamete).
Haploid: half of the total number of chromosomes in a somatic cell. The haploid chromosome set is the set of chromosomes present in a gamete, irrespective of the chromosome number of the species.


That being said, diploid and haploid are not antonyms nor mutually exclusive terms. A cell can be diploid and haploid at the same time. Let's exemplify this with organisms that perform gametic meiosis:

Human beings have diploid somatic cells, with 46 chromosomes. When a somatic human cell perform meiosis, it produces haploid cells which are monoploid. Human gametes are haploid and monoploid.

In wheat (Triticum aestivum), somatic cells are hexaploid, having 42 chromosomes (that is, 6 full sets of 7 chromosomes). When a wheat cell perform meiosis (producing micro and mega spores, and later on gametes), it produces haploid cells which are triploid. Wheat gametes are haploid and triploid.

The same whay, a tetraploid organism would produce, by meiosis, a cell which is haploid and diploid. Thus, depending on the number of chromosome sets in the somatic cell of a given species, you can say that a gamete is diploid (as stated in this other answer).

In a nutshell, a gamete that was produced by meiosis (there are life cycles where the gamete is not produced meiotically) is always haploid, regardless the number of chromosome sets it has (which will determine if it is monoploid, diploid, triploid, hexaploid etc...). 

Sources:


Hartl, D. and Ruvolo, M. (2012). Genetics. 1st ed. Burlingham, Mass.: Jones and Bartlett Learning.
Genetics-notes.wikispaces.com. (2017). genetics-notes - Ploidy. [online] Available at: https://genetics-notes.wikispaces.com/Ploidy [Accessed 1 May 2017].