We started in Banff on August 5 and reached Antelope Wells on Sept 27. When we take this trip again (some day) I think we would likely start in Banff a little later, probably between Aug 10 and 15. My comments are based on only one trip - and we may have had a weird year.
Amy did a fair bit of research using the historical weather records on wunderground.com to figure out rain patterns and essentially decided that there wasn't a reliable enough pattern to pick our dates in order to avoid rain.
Mosquitoes can be a problem and we believe they are worst in June and July. We are not experts about mosquito timing in BC/MT/WY/CO because we don't live there so we are basing this on the GDR and CDT trip reports, and on our own experiences in the Sierra Nevada and the
mid-west. We just hate being around mosquitoes as they make us nutty, so we chose to go later in the year to try to avoid them. We were successful; we really didn't have any mosquito problems until we got into NM.
A possible reason that the mosquitoes were gone is that night-time temperatures had dropped far enough below freezing to kill them. IWe didn't keep notes on this, but we probably had 8-10 mornings with frost in our first 20 days of riding, with night-time low temps during those first few weeks generally between 25 and 30. So you could consider whether you dislike mosquitoes more than you dislike cold weather.
NM gets most of its rain during the monsoon season in July and August. It is hottest prior to the monsoon and May and June are usually the hottest months. The rains cool things off a little and the wildflowers bloom after the rains: we had fantastic wildflowers in NM (although we missed the wildflower peak further north). We had relatively pleasant weather in NM - about 85 degrees, but we don't know if that is typical for late September or if we were just lucky.
We prefer cold weather over hot weather, and we would rather have 30 degree mornings and 70 degree days than day-time temperatures in the 50-90 degree ranges. But, if you stretch too far into September or October then you risk snow in southern CO. We don't mind if it snows a few inches so long as it is still early enough in the season that we know it will melt off in a day or two; just park in your tent and wait it out. But you certainly wouldn't want to be up on a high pass in mid October and have a foot of snow that stays all winter.
The aspen autumn colors are another factor we considered. We were hoping to be there when the colors were in their glory, but we were too early. For this reason, when we take this trip again, we think we'll start a little later.
Look at Ben's short GDMBR trip Colorado video:
Another factor to consider is whether you want to be out in the hunting season. The benefit is that you don't need to worry about being stranded on a remote road without access to help, the downside is that there may be more traffic than in the non-hunting season. We would guess 40% to 70% of the traffic we saw on most USFS roads would not have been there but for the hunting season. We found that the hunters we talked to were kind and generous, and typically were polite drivers, but it does reduce the sense that you are "out there" in a remote place.
The last timing factor was estimating how long our trip would take. We guessed it would take 70 days based on data from the GDR guidebook, but it took only 54 days so we were in CO earlier than we expected and that might be why we missed the aspen colors. We calculated our start date so that we would be crossing from CO into NM on about Oct 1st (we actually crossed a few weeks before that) and we still think an Oct 1 exit from Colorado would be great timing for a trip.
If you have the flexibility to have an open-ended finish date we recommend taking that approach. The people we met on the trail who had to arrive at Antelope Wells on a particular date were driven by that, and couldn't freely choose when to stop and when to have rest days. That seemed to add some stress to their experience which we never had.
Other folks who've ridden the GDR, or are more familiar with the weather patterns on the route, should chime in.