Month-old thread but I have this antenna so I'll throw in some comments...
The answer depends A LOT on how high the GMRS repeater's antenna is. If it's 150 ft or higher you're probably fine.
I have that same UT72G antenna on a file cabinet in a second floor corner home office. Works great. (Around 1.3 SWR on ham bands too.) I used it with a Radioddity DB20G (Anytone 779UV) and power supply on my desk for a while. BTW, decent and TINY moble for $110 (tiny=good for fitting in a nook in the car. Bad for viewing the tiny display).
Pretty flat terrain around here. Here's some results with my UT72G:
15 miles, GMRS repeater @ 60 ft: Can hear traffic but it's really hard to decipher, and I can't hit the repeater even with 20 watts.
18 miles @ 300 ft: Loud and clear with 5 watts, suburbs all the way.
20 miles @ 200 ft: Can hear with substantial noise, but they don't always hear me with 20 watts. The entire metro area (aka lots of RF interference and random FRS/GMRS users) are between me and repeater.
30-35 miles @ 250-450 ft: Can hit these VHF ham repeaters okay with some static, but not any UHF ones. (The GMRS repeater in town at 750 ft might have legs that long though!)
Note that if your antenna has to transmit through your entire house and not just one wall in the direction of the repeater, range will be reduced.
Someone mentioned the N9TAX slim jim antennas. They are well made. I have the ham one for portable use. Hung it in my office to test it, did improve signals a bit over this whip. The MURS/GMRS version of that slim jim would probably work nicely as an attic antenna. Be sure it's hanging straight vertical and not curling up.
