ok,
i think the missunderstanding comes from "Gateway Server".
I personally prefer to talk about Front End Server FES which contains the frontend Gateway component SAP_GWFND needed on the FES.
Yes you can update your NW 7.40 FES with SAP UI TECHNOLOGY 7.50. (alternative SC).
SAP NW GW FOUNDATION 7.40 SP has to be the same SP as SPS of NW AS ABAP 7.40 (SAP UI 7.50 can be any SP so far)
there is also dependency betweean FES NW GW FND 7.40 SP and your backend BES based on NW 7.00 gateway components IW_BEP and IW_PGW.
described in note 1942072 .
For SAP_GWFND 7.40 SP9 (which pretty old SP14 is current) this would be e.g. IW_BEP 200 SP9 and IW_PGW 100 SP06. but it increases if you increase FES GW SP.
Required Minimum SPS for your backend NW 7.00 AS ABAP SPS should remain to be SP18 according to my Solution Manager (dont know a note where this is also described).
I cannot comment about IW_PGW need on the BES. If Fiori is you only reason to have GW for you, this should be described in Fiori installtion guide or landscape guide. or your solution manager should have the dependency.