English Wikipedia says of Ramakkalmedu that “the view is striking at dusk when all these towns in Tamil Nadu are lit up”. A local tourism site puts it more directly still: stay for the dusk views. Both are right about the view and neither mentions the gate.
The two numbers, side by side
The District Tourism Promotion Council, Idukki, publishes the hours of its Ramakkalmedu centre as 08:30 to 18:00, all days, on its own destination page. Computed for the viewpoint's own coordinate and cross-checked against an independent ephemeris to within two minutes, the sun rises on this ridge between 06:00 on 21 June and 06:38 on 1 January, and sets between 18:06 on 21 December and 18:44 on 21 June. Put those together and the arithmetic is blunt: the counter opens roughly two hours after first light every day of the year, and closes before the sun goes down every day of the year. There is no season in which the trick works.
What that leaves you
Plenty, as long as you know it in advance. The plain is at its clearest in the first two hours after opening, before haze builds over the Cumbum valley — every published photograph taken from this escarpment in the middle of the day shows it. Late afternoon gives the best light on the fields and on the escarpment face itself. What you are giving up is the first and last hour, which on a ridge like this are the two that photograph best.
Where the ridge's early and late light actually is
DTPC runs a second centre on the same ridge, at Aamappara — the tortoise rock — and publishes its hours as 06:00 to 19:00. It is 5.8 kilometres away by the road the jeeps use, down to Thovalappady and back up, and it is open for both ends of the day that this compound is closed for. The sensible shape of a Ramakkalmedu day follows from that: the tortoise rock from six, this gate from half past eight, the statue and the hornbill tower through the middle of the day, and be moving down before the counter shuts at eighteen hundred, because the hill road is unlit and mist comes over it in the cold months.
And the rock is free
Ramakkal itself stands outside the DTPC enclosure, 756 metres north of the viewpoint, and costs nothing. The 700-metre climb to it starts beside the Sree Rama and Durgga Devi Temple on the ridge road, not at the ticket gate. If what you came for is the rock and the light rather than the compound, no gate hours apply to you at all — but there is also no railing, no staff and no lighting up there, so the same advice about being down before dark applies twice over.