Agree with the others. RG8/X is fine for 10/11 meter, but your repeater will suffer massive line loss on GMRS. Also only 30 feet high with RG8/X coax will result in horrible repeater performance. You'll be lucky to cover 2 miles. I use LMR400 on a 50 foot run. I had my neighborhood repeater antenna at 30 feet for a while, but coverage wasn't that great. I recently popped it up another 20 feet to 50 feet and i more than doubled my range that's now averaging 10 miles on a 2 watt repeater. Notice that i now am higher than most the trees around vs at 30 feet i was being blocked by trees. The higher you can get that antenna the better your results will be.
You also asked about antenna recommendations. I personally use a copper j-pole tuned to gmrs frequencies. Durable, light, cheap, effective. There are better and more expensive antennas, but for home / non commercial use, these work very well.