There are several ABAP 7.4 Cloud trials that include Gateway (as Andre Fischer mentioned in his blog). Some of them are listed on Developer & Trail editon page. Each of this Cloud trial is free from licensing prospective, but has associated Cloud usage costs (differ depending on resources used). The cheapest are any db (not HANA) options like: AS ABAP incl. BW 7.4 SP8 on SAP ASE 15.7 ($0.39/h) and BW 7.4 SP2 on SAP MaxDB 7.9 ($0.51/h). What is missing in the list is the most cost effective option: SAP NetWeaver Application Server ABAP 7.4 on SAP MaxDB [trial edition] ($0.26/h). Cloud image for this tiral does not come with fully configured Gateway, but it easy to fix maintaining SAP NetWeaver Gateway Settings.
AS ABAP incl. BW 7.4 SP8 on SAP ASE
BW 7.4 SP2 on SAP MaxDB 7.9
SAP NetWeaver Application Server ABAP 7.4 on SAP MaxDB
Trx.SPRO SAP Customizing Implementation Guide -> Activate Business Functions -> SAP NetWeaver -> Gateway Service Enablement -> Backend OData Channel -> Connection Settings to SAP NetWeaver Gateway -> SAP NetWeaver Gateway Settings
Once Gateway is configured you can start you journey of exploring SAP NetWeaver Gateway. Here are two documents I recommend to follow to learn cleation of your own OData Services: