Complete NetSuite amateur here who is really struggling to understand the system. Does anyone know how to run a transaction report filtered for multiple GLs? Is there an "or" function in report filters? When I seperate the GL numbers by commas, it says "no match"

Which specific report? You can also achieve that through a Transaction Saved Search. Lists > Search > Saved Searches then Transaction. Under Criteria, set these: Main Line = False, Account (desired account/s to search), etc. Under Results, select the desired fields.