Are there any ways for Green to Gold soldiers to pay for tuition other than using their GI Bill?

There are three G2G programs. Active Duty Option, Scholarship, and Non Scholarship option. We're leaving out Hip Pocket program because it's not an actual option. Each has its own requirements. Green to Gold is just a fancy term for ROTC. For the Scholarship option the Soldier receives a scholarship for tuition. For the non scholarship option the Soldier uses their GI Bill or if they join the NG they can enroll in SMP and use TA. ADO are required to use their GI Bill or pay out of pocket so they shouldn't apply till they've fulfilled the three years required to use 100% of their GI Bill or the six years if they used student loan repayment. The scholarship and non scholarship options separate the Soldier from active duty to Cadet Command and the ADO Soldiers stay on AD. ADO Soldiers have to finish their degree in two years or less, so it's not for Soldiers with zero college. Chances are the Soldiers you knew in ROTC were scholarship or non scholarship cadets

It depends on which GtG program they use. There is only one non-scholarship option that would need the use of GI bill. The others are the 2, 3, 4 year scholarship programs, the division commanders hip pocket scholarship program and the 2 year active duty option. To find the best one for your soldiers is a duckduckgo search away...
https://www.goarmy.com/careers-and-jobs/current-and-prior-service/advance-your-career/green-to-gold.html