I believe it's worthwhile for an experimental glider to transmit adsb-out, even if only using an uncertified gps, because it will make your glider show up more accurately on Flarm receivers even if you don't carry flarm yourself (adsb-out targets show up on flarm with exact location and altitude). You should also show up for equipped power aircraft even if out of ATC radar contact.

The main reason to pay for a certified gps to send adsb-out would be if you also plan to install adsb-in and a traffic display. I still don't see a low-cost low-power adsb-in system for a glider, although I'm sure they will come. The FlightBox adsb-in device appears to draw 1.3A at 5V, ie probably 0..7A at 12V, which would be too much for my glider battery. Also, I would want a sunlight-readable display (I don't find iphones etc to be sunlight readable).

I don't plan to install flarm in my glider, because I think transponder+adsb will soon be the better choice.