mod_rewrite to direct some requests to a new directory - mod-rewrite

Question

I have a website with a frontend and a backend that all run through index.php. To access the back end you would go to http://www.mysite.com/Admin. This will still run through index.php but then connect to the database and go to the correct directory. The backend manages donation information that is collected on the site.
I want to change the front end to WordPress but still collect donation information in the original database and view it from the backend.
Rather than rewrite all the code, I was wondering if there was a way I could use mod-rewrite and place the WordPress files in a separate directory. This way any request that included /Admin as the start would go through the original index.php and any other request would redirect to directory with WordPress.
I originally tried:
RewriteCond %{REQUEST_URI} !^/Admin
RewriteRule ^(.*)$ gpf/$1 [L]
but a request for /Admin just gives me a 404 error.
Thanks in advance for any help.
This is the full .htaccess file
RewriteEngine On
Options -Indexes
# Serve 'gate' as php
<Files gate>
SetHandler application/x-httpd-php
AcceptPathInfo On
</Files>
FileETag MTime Size
AddCharset UTF-8 .xml
AddType video/mp4 mp4 m4v
AddType audio/mp4 m4a
AddType video/ogg ogv
AddType audio/ogg ogg oga
AddType video/webm webm
# redirect all domains (.net, .com, .biz, etc) to .org
RewriteCond %{HTTP_HOST} !^www\.mysite\.org$ [NC]
RewriteRule ^(.*)$ http://www.mysite.org/$1 [R=301,L]
# Route search
RewriteRule ^search$ /search/ [R,NC,L]
RewriteCond %{REQUEST_URI} !^/Admin
RewriteRule ^(.*)$ gpf/$1 [L]
# Route domain plus slash correctly
RewriteRule ^$ /gate/ [L]
# All requests: serve _gate_ as php. 1st arg is requested URL qs preserved
RewriteRule ^([0-9A-Z].*) /gate/$1 [QSA,L]
RewriteRule ^rss\.xml /gate/rss.xml [QSA,L]
# PHP directives - should comment out on production server
php_value display_errors 1
php_value display_startup_errors 1
AddOutputFilterByType DEFLATE text/html application/x-javascript text/css text/javascript text/plain
<IfModule mod_expires.c>
ExpiresActive On
# Manually change filenames for static images. CMS changes names in dyn
ExpiresByType image/gif "access plus 9 months"
ExpiresByType image/jpg "access plus 9 months"
ExpiresByType image/jpeg "access plus 9 months"
ExpiresByType image/png "access plus 9 months"
# Rare changes. manually change filename
ExpiresByType image/x-icon "access plus 9 months"
ExpiresByType image/vnd.microsoft.icon "access plus 9 months"
# PDFs should never change. CMS renames in dyn
ExpiresByType application/pdf "access plus 6 months"
ExpiresByType application/x-pdf "access plus 6 months"
# Frequent changes. ASFU dynamic rename
ExpiresByType text/css "access plus 6 months"
ExpiresByType text/javascript "access plus 6 months"
ExpiresByType application/x-javascript "access plus 6 months"
</IfModule>

Answer 1

Try adding an additional condition:
RewriteCond %{REQUEST_URI} !^/Admin
RewriteCond %{REQUEST_URI} !^/gate
RewriteRule ^(.*)$ /gpf/$1 [L]
And move the rules after all of the /gate/ rules.