Yep, ramps work in GC just fine for me.
It seems like you're using Intervals.icu to build the workouts which is what I do so here's my workflow:
1 - Create workout in Intervals.icu using the required conventions
2 - Export/Download from Intervals.icu in .ERG format
3 - 'Import Workout From File" that ERG file into GC's Train tab
Done
If the ramps aren't appearing correctly you can open the 'Properties' of the workout and add a hyphen (-) to the power data section between the desired Power High and Power Low target.
For example, below is where I have ramped a rest/cooldown from 143W to 114W over 10 minutes:
(I also like to add an "L" to the end of interval lines to create laps so GC rings the lap bell)
Hope this helps