How Do Linguists Know That the Word Gift Came From Old Norse?

Linguists know that the word gift came from old norse and not old english. The original germanic g sound gu shifted to a why sound in old english when the sound appeared before the front vowes, specifically, e and i. So linguists know with a fair amount of certainty that the English words giavan and gavo were pronounced as yavan and yvo by the time of late old english. And we'll see how those sound changes actually help to reveal the large number of norse words in english.