I'm not 100% sure I understand your post, but I had a look at the info myself (I might have to go back to Seoul on business, like it or not, so I was keen to find out about the new railway) ... the new railway will take you from Incheon to Gimpo (3200 won, 33 minutes) and then you transfer to the subway and go the rest of the way. Was that what you meant, your hotel is close to Gimpo and it only takes 5 minute from there? Or the 95 minutes is the whole journey (33 mins plus 5 mins transfer plus 55 mins or so on the subway?).
Anyway - it sounds doable. The Airport Limousine bus that I used to take (to the Central Air Terminal near Coex) used to cost 11,000 won (it was a NICE bus!) but there might be cheaper "local" buses, not sure. It went every 10-20 mins or so, so pretty convenient and took about 90 mins (coex is on the other side of the city from the airport -if you hotel is closer then would be cheaper and faster on whatever bus you choose).
I have travelled twice on the subway. It's not simple. As I recall you have to purchase little tokens at a machine before you go in (a local helped me figure this out, you will probably find more helpful signs if getting on at the Gimpo airport station!). But it was clean and well signposted. VERY crowded during rush hour - and it's a HUGE system so does take quite some time to get from one side to the other. But very usable - a good way to get around Seoul I thought.
Anyway - whatever you decide I'd be interested in feedback when you return.....