A minimum bet is required to be eligible to win it though.
If your Grand Jackpot indicator is greyed out, or locked, reads "Play Grand" or "bet up to play", then you need to increase your bet to activate the Grand Jackpot feature.
 
If the Jackpot reads:
"Play Grand" - the Grand Jackpot is NOT unlocked
"Grand" - the Grand Jackpot IS unlocked and can be won
 

If the Jackpot reads:
"Bet up to Play" - the Grand Jackpot is NOT unlocked
"Grand" - the Grand Jackpot IS unlocked and can be won

If the Jackpot shows as locked - the Grand Jackpot is NOT unlocked

If the Jackpot reads "Grand" - the Grand Jackpot IS unlocked and can be won