Check the sliding door bearing. It maybe due for replacement. I have no problem with the sliding door, one is very silent another is noisy because the bearing is due for replacement.

Was it like this when the gate and sliding motor is new? Mine was the same, and slowly make noise. The technician disable the sliding motor and pull, it show us the bearing is worn and motor now work much harder.  But still workable.

Sliding is always slow.

Can you try unlock the motor and manually push. If its easy push mean is the motor problem. If still hard, most likely the roller inside the gate is rust or failing.

That is not a problem, most of the autogate mainboard has this function. But you mentioned 20 feet gate?
Local cheap arm might not be able to last long for this length.

DC Moto and Gengate system actually is the same function only the casing is design differently. It's like comparing Honda Accord with Proton Perdana new model. But who would you go for  if you got the money for quality, of course the actual manufacture that is Honda, right. Don't get me wrong but I prefer actual then OEM. I myself use DC Moto for 1 year++ since I've change my gate to folding type and it's working find for me without any problem that why I'll keep on installing DC Moto for the quality.

Check our facebook dblcss.klg 21st January 2014. That's my own gate with DC Moto 925W. I believe in what I carry because I'm using it myself and confident enough to tell my customer DC Moto is great, durable and reliable.

And do check out another we done at Kota Damansara on 21st January 2015. Gate 8 feet tall and 12 feet long for each side. As I've told the customer as long the gate is install properly there is no problem for DC Moto to push open and close the gate. Max for DC Moto is 15 feet long.

Anyway all the best

Recommended is 15 feet one side. So both side is 30 feet gate.
If bigger and heavier then have to go for italy hydraulic motor liao