For a 1 Gbps link between two Aruba CX switches, the appropriate SFP port type is an SFP (Small Form-factor Pluggable). SFP+ and SFP28 are used for higher speeds such as 10 Gbps and 25 Gbps, respectively. QSFP+ is for even higher speeds (40 Gbps).

SFP supports 1 Gbps (1000BASE-X)

SFP+ supports 10 Gbps

SFP28 supports 25 Gbps and above

QSFP+ supports 40 Gbps and above

Therefore, the correct port type for a 1 Gbps link is SFP.

Aruba CX Transceiver and Port Type Compatibility Guide

HPE Aruba Networking Switch Hardware Documentation

Aruba Networking Best Practices for Link Speeds