How to model Involute Gear in Solidworks?
these are the steps...

Step 1:
First of all we need to add some equations... so Tools > Equations >

Step 2:
We add the following equations... (module and No. of teeth is up to you)

Step 3:
After we add all equations, we click ok and then we start a new sketch on front plane... We create a construction line vertical...(Random length)

Step 4:
Then we create a horizontal construction line at the length of the D0.. and then we draw a circle.. We make the circle defined by linking the dimension with the equation..

Step 5:
Next we create another construction line with an angle of a(degrees)...

Step 6:
We create another one on the other side and make them colinear. Then we draw another one from the cinter of the circle and make it perpendicular...

Step 7:
We draw a circle from the center to that point. We then draw 5 construction lines from the center of the circle dividing the arc into 5 pieces...

Step 8:
After, we divide the line above into 5 pieces with points...

Step 9:
And we make the sections equal.. after that, we link the section dimensions, with the arc sections below...

Step 10:
Next we draw construction lines, from every point of the arcs, and make them tangent to the circle... dimension them as in the picture...

Step 11:
Same thing for the back side...

Step 12:
spliting...

Step 13:
tangent lines...

Step 14:
next we create top circle of the gear... and link the dimension to the equation shown in the picture...

Step 15:
same at here...

Step 16:
select spline, and join the end points of the contsruction lines...

Step 17:
Join the lower end of the spline with the center of the circle with a line..

Step 18:
create a construction line in the point shown in the picture..

Step 19:
mirror...

Step 20:
convent entities for the 2 circles..

Step 21:
trim...

Step 22:
extrude the circle...

Step 23:
then the tooth..

Step 24:
add fillet linked with the following equation...

Step 25:
Circular patern linked to Z

Step 26:
And Voila.... a fully defined involute gear...