Which GPU coins are worth mining other than ETH now?
How much investment it needs for 1 dash masternode and how much return per month it can give?
There are a lot of GPU mineable coins, you will need to look for the ones which are ASIC resistant so you won't be forced to compete with ASICs when mining with GPUs. If I had some mining rigs I would mine Monero, maybe some ZCash too.
Regarding Dash: 1 Dash masternode is, at current price, $112K and it will bring you $624 / month according to
https://masternodes.online/currencies/DASH/