I never tried RPM in gym, but there is nothing versus real-time hill climb in front of you.

In gym, you set the knob to "tighten up" but you can always slow down rpm or reduce tightness (cheat ownself while the trainer does not notice  ). In real life facing a stretch going uphill for 500m, you got no choice but to go all the way. You can't cheat anybody but yourself. Unless stop, get down and push the bike.
Add a strong blowing head-wind when you going uphill ... 

You can try build your muscle and stamina, but need to practise on real course from time to time.

I used to ride mountain bike daily 25k-35km on-road & off-road (not using roadbike) and now looking to start back with foldie stuff. 

Now still unsure what to buy 20" wheels:
option 1: budget <RM1.2k like dahon vybe but no FD
option 2: budget RM2-3k prefer with both FD and RD (2x8 or more) is enough. I really like that dash D18 specs, or Speed/Mu D8, Vitesse D8


My ex-mountain bike is 3x7 = 21 speed. I prefer with FD to play with big (speed) and small (climb) crank. Rear cassette with 7 or more speed is more than enough, if front crank has two gears already.