Characters are on a grid where every square is 5 feet on each side.
The description of the gust of wind spell says:
A line of strong wind 60 feet long and 10 feet wide blasts from you in
a direction you choose for the spell’s duration.
How do I center a 10-foot-wide line originating from the caster on a grid made out of 5-foot squares?
Best Answer
Choose an intersection as the point of origin.
The rules for areas of effect on a grid require that you choose an intersection of squares or hexes as the point of origin. From the Dungeon Master’s Guide (pp. 250-251), in the rules for using miniatures, it says the following about areas of effect:
So you would choose one of the corners of the space you are occupying as the point of origin for gust of wind.
Xanathar’s Guide to Everything (pp. 86-88) has some additional guidance in the section "Areas of Effect on a Grid", and even includes examples of how to adjudicate a 10-foot wide line on a grid: