Gee I posted the code for you in the above post. Try index.php. And if all else fails if you make regular backups like everyone should then just do a backup install to a clean version. Also might help to get your isp to update there version of php. GB version 2.3.1 shouldnt be affected my the exploit if php is updated. Unless someone just outright hacked your username and password. In which case you must use real simple ones.
ROFLMAO Iam sorry I know its not funny but I found it just by looking at your source code. Look for that in your files. By the way here is there Ip information and there ISP server In Finland.
http://www.mbnet.fi/mbinternet/ Name: www.mbnet.fi
IP Address: 194.100.161.11
Location: Vantaa (60.178N, 24.933E)
Network: RIPE-CBLK2
<div align="center"><br />
<div align="center" style="width: 790; height: 2000"><br />
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<p align="center"><font color="red" size="10"><b>This Site Hacked By <br />
BLaCk ScorPioN <br />
<p align="center"><img border="0" src="http://koti.mbnet.fi/sna/mortalkombat/images/mk5char/scorpion.jpg" width="300" height="300"></p><br />
¤BLaCk ScorPioN ¤</i></font></p><br />
<p align="center"><font color="red"><span style="BACKGROUND-COLOR: blck"><br />
<p align="center"><font color="red" size="6"><i><br />
My Team rEd ScorPioN & ELsa7er hackEr Muslim<br />
<p align="center"><font color="red" size="5"><i><br />
Website U.S WwW.hakr.tk</span></font></p><br />
<p align="center"><font color="red"><span style="BACKGROUND-COLOR: #000000"><br />
<p align="center"><font color="RED">black_scorpion_xp10@HotMail.com</font></p><br />
<p align="center"><font color="red"><span style="BACKGROUND-COLOR: #000000"><br />
<p align="center"><font color="RED"><br />
My Masseaeg>>> Fuck You Admin<br />
I'm Not Sorry Hacked Your Site BY<br />
Red Scorpion Taem We Arab Hackers We Well Fuck Evreyone From Usa Or Israel<br />
Fuck Usa And Fuck Israel<br />
lOl>>>>>>><br />
¤Mr 3aDeL_hackEr¤</font></p>
You need to put them under your public_html folder. And you do realize that GB 1.5 is very old. The new version is now 2.3.1 Plus you will need to install a database.