If swr is halfway decent it will work just fine... i have been operating a 25w anytone 778uv base setup(just upgraded to 50w anytone 5888uv, which seems to output about 42w on gmrs) off a "bingfu" 30" mobile dual band antenna literally electrical taped to a piece of conduit with a radial ground plane, and can operate a repeater 22 miles away on a daily basis, on a $30 antenna showing approximately a 1.9-2.2swr on gmrs frequencies.... and i can hit the repeater better than someone right up the road from me also running 50w, thru a commercial 6' comet for gmrs, with a 1.4 swr across the repeater band....
All that said, TLDR..... get the free antenna and see what it can do.... worst that can happen is the FREE amtenna doesnt work well, and you end up padsing it along to someone else who can use it....