you can look here and decide if you need to hire an electrician.
the calculator sez you will need #4 copper conductor for a run of 125 feet at 30 amps. 3% line loss.
read big bucks!
you do not want to scrimp on the wire because the life of your airconditioner depends on "good" voltage. i would not reccomend running your airconditioner on a sub standard circuit because you can burn the compressor out.
the gfi tripping is not good!