Bill,
I don't know about Mode-S for Canada but if you plan to fly into Canada I see a bigger issue with your plan and that is with regard to your ADS-B out. The GDL-88 uses the 978 MHz "Universal Access Transceiver (UAT) frequency. As you correctly noted this is fine for use under FL180. However... the UAT frequency has not been adopted by any country other than the US.
The true "universal" and ubiquitous world-wide standard for ADS-B out is 1090-ES or 1090 MHz "Extended Squitter" which piggybacks on the existing signal from a 1090 MHz transponder.
The GDL-88 will RECEIVE both 978 MHz UAT and 1090 MHz ES for ADS-B "IN" to display traffic on a compatible display but the GDL-88 only provides 978 UAT for the ADS-B "OUT". This means that you will be limited not only to staying under FL180 but also to staying within the continental United States.
Most avionics manufacturers have a pretty good write up on this. You can read about this on Garmin's website here ->
http://www.garmin.com/us/intheair/ads-b/squit/Notice on that page I reference above under the heading "ES Takes You Higher" it says (in part):
"...while UAT is limited to aircraft that will operate no higher than 17,999 ft MSL. So far, no country except the U.S. has accepted the 978 MHz UAT format for their ADS-B datalinks. Consequently, pilots who want to fly outside the U.S. — or operate at or above FL180 — will need the 1090 ES transponder for ADS-B “Out”..." (emphasis added in bold)
Thus if you plan to fly into Canada you will need a 1090-ES Out solution. If you are putting in a new Mode C transponder such at the Garmin GTX-327, I'd recommend you instead look at the GTX-330ES. It's more expensive but it's Mode-S and has the 1090-ES out built right in. It does require a WAAS GPS source which the GDL-88 already has built right in. The GDL-88 can still give you the "In" capability dual-band on both frequencies.
I would talk to your avionics shop / installer about where you plan to fly because once the ADS-B curtain falls (and that's happening earlier than 2020 in many countries) you won't be able to get outside the US with a GDL-88 978-UAT Out solution.
Good Luck,
- Charles