I assume you have looked up the dates of the marriages on Freebmd website
1st marriage William Gillis to Ann Davis DecQ 1858
2nd marriage William Gillis to Jane Evans MarQ 1879
Both marriages as you say were registered under the district of Hay.
you have a couple of choices now to obtain further information
1. send off for the marriage certs - the office holding the records is
The Register Office, Powys County Hall, Llandrindod Wells Powys LD1 5LG
Cost of each cert 7.00 pounds
you will need to quote the above details
2. send off to GRO in Southport
Here is an online link for further information www.gro.gov.uk/gro/content/
3. Look for the entry in the parish registers - you suggest Llandeilo Graban was where he was living. This may come under Powys Record Office- I am not sure
You can email them on archives(at)powys.gov.uk
They can tell you what holdings they have and how much it would cost to look up the records if you cannot do so in person
4. Contact the National Library of Wales and see if they will look up the parish register for you.
Unfortunately there is no way anyone can look up the civil registrations of birth, marriage or death for you only the indexes. The only way then is to purchase the relevant certificates.