The reviews of the Mono Gaps here on eham seem decent.
eHam reviews never reflect actual performance. They are like asking a Ford driver how he likes his Ford, or a GM driver how he likes his GM.
If you look at a real review, where the antenna is compared to other antennas, the Gap vertical is OK on some bands and terrible on others.
If you look at QST's review, where they compared a Gap to a multband trap vertical, the multi band trap vertical won.
I had a Gap vertical in the 90's, and a 6BTV was equal on the Gap's best bands and blew the Gap away on some bands. The Gap was 10 dB down from a cheap MFJ 80-40 vertical on 80 meters, and 5 dB down on 40!
You might get a copy of K7LXC and N0AX book about HF Vertical comparisons, and look at performance. For 40-10 meter avarage gain, the only antenna worse than the Gap is the MFJ-1798.
For 80-40 average gain, the Gap is -11.2 dB, or 6 dB worse than a 6BTV.
For 20 on up the Gap isn't so bad, but on lower bands it isn't good. Both QST and the HF Vertical Comparison bear this out.
Be very cautious of any lower band HF vertical that claims no ground is required.