You have a couple of options:
Go into the gui settings area, click on trading pairs, click on the pair generating the message and add BOUGHT_PRICE as the label with the appropriate price you paid as the value.
OR
Open config.js, find the pair and add the BOUGHT_PRICE in the overrides section, don't mess up the formatting of the file though, safer to do it in the gui
"pairs": {
"bittrex": {
"BTC-ETH": {
"strategy": "gain",
"enabled": true,
"override": {
"BOUGHT_PRICE": 0.03160000
}