Parker Moreland did an excellent analysis on measuring g with a photogate and suggests a procedure for minimizing error. This article also includes a formula and chart for calculating a local value for gravitational acceleration.

“Improving precision and accuracy in the g lab.” Parker Moreland. The Physics Teacher, Sep 2000.

You can also get a theoretical local value for g from WolframAlpha,
Replace Beaverton, OR in this example with your own location:,+OR

See also:
How can I use a Vernier Photogate to measure g?
How does Logger Pro calculate velocity and acceleration from photogate data?